Yup, it's working on my site now; it was simply that I didn't modify the postbit_legacy template. Thanks for pointing that out, Chroder.
One little problem, though: sometimes when I quote a post, old quotes from completely different threads I multiquoted previously will show up in the message box. Nothing major, but somewhat annoying - and confusing - when it happens. I assume there is a cache or something somewhere that's not being cleared when you move on to another thread.