In the end, I just hit the "get allocations" button like crazy until something big appeared, fixed that, then repeated.
Although it felt a bit stupid, I believe this took less time than faffing around with manual DDMS dumps, manual running of the hprof converter tool (what a joke that is - "hey, we broke the standard. And, uh, we're not going to fix it. But here's a commandline tool you can use to un-break what we broke! have fun!"), etc.
(NB: I recently tried to enable the Eclipse plugin that's supposed to handle live hprofs of all kinds - it's too broken, and there are no docs, just a couple of long boring youtube videos. I gave up at that point