So I've got a hung emacs that I have attached to in a debugger. I opened a new emacs to use report-emacs-bug; following the instructions I was going to include the output of 'bt full'. Except that it's about 17500 lines long...
I took a look at the backtrace as well, and of the 3800 calls on the list, ~3700 of them are from either mark_object or mark_vectorlike.
I'm not sure what else to do (I'm not familiar with emacs c code at all), but I can keep the hung emacs running in the debugger pretty much indefinitely if that's useful.