After a bit of fiddling, I tried David's solution in #18 to cubbtech's request from #17. While it only required a few lines of code (maybe 20 or so all told), the changes were scattered about several files. Given the hacky nature of the changes to my already-patched Drupal 6 version I won't submit a patch here.
Nonetheless, here's a rundown of how I got it to work for multiple-select taxonomies:
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