Reporting back:
I had no trouble converting Serpentine's version - thanks so much

It wasn't quite usable as is - there were a lot of files with identical names in different directories that got overwritten, there was some css formatting that got text cut from the mobi, and I wanted to get the order more reasonable.
But, knowing the conversion was doable, I wrote something short that just yoinked out the correct div in every page (and fixed links and stuff), and everything else was straightforward and worked fine. The final epub-->mobi took about 400 minutes... but worked fine.
Thanks so much for all the help