diff --git a/collects/scribble/base-render.rkt b/collects/scribble/base-render.rkt
index cbc6da6c..e09318db 100644
--- a/collects/scribble/base-render.rkt
+++ b/collects/scribble/base-render.rkt
@@ -288,7 +288,11 @@
     (define/public (transfer-info ci src-ci)
       (let ([in-ht (collect-info-ext-ht ci)])
         (for ([(k v) (collect-info-ext-ht src-ci)])
-          (hash-set! in-ht k v))))
+          (hash-set! in-ht k v)))
+      (set-demand-chain-demands! 
+       (collect-info-ext-demand ci)
+       (cons (collect-info-ext-demand src-ci)
+             (demand-chain-demands (collect-info-ext-demand ci)))))
 
     ;; ----------------------------------------
     ;; document-order traversal
@@ -386,10 +390,11 @@
     ;; ----------------------------------------
     ;; global-info collection
 
-    (define/public (collect ds fns fp)
+    (define/public (collect ds fns fp [demand (lambda (key ci) #f)])
       (let ([ci (make-collect-info fp
                                    (make-hash)
                                    (make-hash)
+                                   (make-demand-chain (list demand))
                                    (make-hasheq)
                                    (make-hasheq)
                                    null
@@ -407,6 +412,7 @@
                    (collect-info-fp ci)
                    (make-hash)
                    (collect-info-ext-ht ci)
+                   (collect-info-ext-demand ci)
                    (collect-info-parts ci)
                    (collect-info-tags ci)
                    (if (part-tag-prefix d)
@@ -911,3 +917,12 @@
     ;; ----------------------------------------
 
     (super-new)))
+
+
+;; ----------------------------------------
+
+(define-struct demand-chain ([demands #:mutable])
+  #:property prop:procedure (lambda (self key ci)
+                              (for/or ([demand (in-list (demand-chain-demands self))])
+                                (demand key ci))))
+
diff --git a/collects/scribble/core.rkt b/collects/scribble/core.rkt
index 966e5606..109ccd43 100644
--- a/collects/scribble/core.rkt
+++ b/collects/scribble/core.rkt
@@ -6,7 +6,7 @@
 
 ;; ----------------------------------------
 
-(define-struct collect-info (fp ht ext-ht parts tags gen-prefix relatives parents) #:transparent)
+(define-struct collect-info (fp ht ext-ht ext-demand parts tags gen-prefix relatives parents) #:transparent)
 (define-struct resolve-info (ci delays undef searches) #:transparent)
 
 (define (part-collected-info part ri)
@@ -34,8 +34,14 @@
                (collected-info-parent (part-collected-info part ri))
                ri key)]
         [else
-         (values (hash-ref (collect-info-ext-ht (resolve-info-ci ri)) key #f)
-                 #t)]))))
+         (define ci (resolve-info-ci ri))
+         (define (try-ext)
+           (hash-ref (collect-info-ext-ht ci) key #f))
+         (values
+          (or (try-ext)
+              (and ((collect-info-ext-demand ci) key ci)
+                   (try-ext)))
+          #t)]))))
 
 (define (resolve-get/ext? part ri key)
   (resolve-get/ext?* part ri key #f))
diff --git a/collects/scribble/html-render.rkt b/collects/scribble/html-render.rkt
index 016fb29d..c97037b1 100644
--- a/collects/scribble/html-render.rkt
+++ b/collects/scribble/html-render.rkt
@@ -1483,11 +1483,12 @@
 
     (define/override (include-navigation?) #t)
 
-    (define/override (collect ds fns fp)
+    (define/override (collect ds fns fp [demand (lambda (key ci) #f)])
       (super collect 
              ds 
              (map (lambda (fn) (build-path fn "index.html")) fns)
-             fp))
+             fp
+             demand))
 
     (define/override (current-part-whole-page? d)
       (collecting-whole-page))
diff --git a/collects/scribble/xref.rkt b/collects/scribble/xref.rkt
index 4b53608b..a6ed768d 100644
--- a/collects/scribble/xref.rkt
+++ b/collects/scribble/xref.rkt
@@ -34,16 +34,23 @@
 (define-namespace-anchor here)
 
 (define (load-xref sources
+                   #:demand-source [demand-source (lambda (key) #f)]
                    #:render% [render% (html:render-mixin render%)]
                    #:root [root-path #f])
   (let* ([renderer (new render% [dest-dir (find-system-path 'temp-dir)])]
          [fp (send renderer traverse null null)]
-         [ci (send renderer collect null null fp)])
+         [load-source (lambda (src ci)
+                        (parameterize ([current-namespace
+                                        (namespace-anchor->empty-namespace here)])
+                          (let ([v (src)])
+                            (when v (send renderer deserialize-info v ci #:root root-path)))))]
+         [ci (send renderer collect null null fp
+                   (lambda (key ci)
+                     (define src (demand-source key))
+                     (and src
+                          (load-source src ci))))])
     (for ([src sources])
-      (parameterize ([current-namespace
-                      (namespace-anchor->empty-namespace here)])
-        (let ([v (src)])
-          (when v (send renderer deserialize-info v ci #:root root-path)))))
+      (load-source src ci))
     (make-xrefs renderer (send renderer resolve null null ci))))
 
 ;; ----------------------------------------
diff --git a/collects/scribblings/scribble/core.scrbl b/collects/scribblings/scribble/core.scrbl
index 7ded2d58..0c03eb8c 100644
--- a/collects/scribblings/scribble/core.scrbl
+++ b/collects/scribblings/scribble/core.scrbl
@@ -1193,7 +1193,9 @@ Returns the width in characters of the given @tech{content}.
 Returns the width in characters of the given @tech{block}.}
 
 
-@defstruct[collect-info ([ht any/c] [ext-ht any/c] [parts any/c] 
+@defstruct[collect-info ([ht any/c] [ext-ht any/c] 
+                         [ext-demand (tag? collect-info? . -> . any/c)]
+                         [parts any/c] 
                          [tags any/c] [gen-prefix any/c] 
                          [relatives any/c] 
                          [parents (listof part?)])]{
diff --git a/collects/scribblings/scribble/renderer.scrbl b/collects/scribblings/scribble/renderer.scrbl
index 4d058634..57a3b446 100644
--- a/collects/scribblings/scribble/renderer.scrbl
+++ b/collects/scribblings/scribble/renderer.scrbl
@@ -127,12 +127,22 @@ information on the @racket[dests] argument.}
 
 @defmethod[(collect [srcs (listof part?)]
                     [dests (listof path-string?)]
-                    [fp (and/c hash? immutable?)])
+                    [fp (and/c hash? immutable?)]
+                    [demand (tag? collect-info? . -> . any/c) (lambda (_tag _ci) #f)])
            collect-info?]{
 
 Performs the @techlink{collect pass}. See @method[render<%> render] for
-information on the @racket[dests] argument. The @racket[fp] argument
-is a result from the @method[render<%> traverse] method.}
+information on the @racket[dests] arguments. The @racket[fp] argument
+is a result from the @method[render<%> traverse] method.
+
+The @racket[demand] argument supplies external tag mappings on demand.
+When the @racket[collect-info] result is later used to find a mapping
+for a tag and no mapping is already available, @racket[demand] is
+called with the tag and the @racket[collect-info]. The @racket[demand]
+function returns true to indicate when it adds information to the
+@racket[collect-info] so that the lookup should be tried again; the
+@racket[demand] function should return @racket[#f] if it does not
+extend @racket[collect-info].}
 
 @defmethod[(resolve [srcs (listof part?)]
                     [dests (listof path-string?)]
diff --git a/collects/scribblings/scribble/xref.scrbl b/collects/scribblings/scribble/xref.scrbl
index e5ce9e2d..9f7d0f08 100644
--- a/collects/scribblings/scribble/xref.scrbl
+++ b/collects/scribblings/scribble/xref.scrbl
@@ -20,6 +20,9 @@ by @racket[load-xref], @racket[#f] otherwise.}
 
 
 @defproc[(load-xref [sources (listof (-> any/c))]
+                    [#:demand-source demand-source 
+                                     (tag? -> (or/c (-> any/c) #f)) 
+                                     (lambda (_tag) #f)]
                     [#:render% using-render% (implementation?/c render<%>)
                                (render-mixin render%)]
                     [#:root root-path (or/c path-string? false/c) #f])
@@ -30,6 +33,11 @@ produce a serialized information obtained from @xmethod[render<%>
 serialize-info]. If a @racket[sources] element produces @racket[#f],
 its result is ignored.
 
+The @racket[demand-source] function can effectively add a new source
+to @racket[sources] in response to a search for information on the
+given tag. The @racket[demand-source] function returns @racket[#f]
+to indicate that no new sources satisfy the given tag.
+
 Since the format of serialized information is specific to a rendering
 class, the optional @racket[using-render%] argument accepts the
 relevant class. It defaults to HTML rendering, partly because