Haven't a clue--I should think XML would help with long documents but no
idea how that is going to work out in practice. Anybody who does know is
under NDA not to tell you, by the way, so this type of question doesn't
usually get much of a direct answer.
Current links re workarounds, just in case you haven't seen them:
IncludeText Fields can partially substitute for the Master Document
feature‹for an introduction to them, see here: http://word.mvps.org/FAQs/TblsFldsFms/includetextfields.htm
(hit refresh a few times in Safari, or use a different browser)
Word experts generally advise combining long documents into one file, if
possible, and you will find more information on controlling those big files
here: http://daiya.mvps.org/bookword.htm