For a start, take a look at this article on Peter Grainge's site. It outlines some of the issues and suggested methods. You are bound to have some element of manual jiggery pokery if you decide to go down this line. That said, Adobe have not announced plans to drop support for RH for Word and even if they did, you could still use RH7 for a long time yet. Maybe you'd want to look at the Technical Communication Suite. You'd have to convert your Word docs to FrameMaker docs but it may prove less troublesome.
Just a thought!
Don't know how useful my question is now, given that you posted over a year ago.
Do your RH X5 Word source docs use styles? If so, take a look here, and I detail how I have been able to import from RH X5 for Word to RH 7 HTML. (I'm currently still working on a leftover issue with Keywords and Subkeywords.)
One thing I have done which probably helped: while applying new standards to our documentation, we have removed almost all images (screen shots, etc.), and have only retained those images which don't have a text equivalent. The drastically reduced the import file size of my Word source docs. All my images were referenced via TrueCode, but upon importing to RH 7 HTML, there was so much less to be processed by RH 7 HTML, that the import went much more easily and quickly.
I hope this helps,