OK, it was this line in the ui.ini that was screwing me up:
I changed it to look in the MAME dir on my NTFS drive and the hang went away. This is a pretty odd thing though, I don't have a ui.ini anywhere else, so I don't understand why running .mame64 from within ~/dev/mame didn't reproduce the hang as well. I would imagine it was referring to the same ~/.mame/ui.ini file.
Not sure if I uncovered something that will help the community at large, or if this is something unique to the way I set MAME up. But as soon as I pointed categorypath to a valid location, mame was happy.