The closest I've come to an answer is the LINK field code:
{ LINK ClassName "FileName" [PlaceReference ] [Switches ] }

The FileName is obvious, and for switches I may or may not use /a, but I'm
not sure what should be in the ClassName and PlaceReference fields.

For PlaceReference, the only examples I could find were for linking in Excel
cells, but I don't know the format for inserting figure or chapter header
references.

Still searching for an answer...