OK, so I got something working by ditching this method and using this instead:
http://stackoverflow.com/questions/5109336/mapview-in-a-fragment-honeycombI'd like to be able to just use a MapView in FragmentActivity like your method allows for, but it was hell trying to get it to work and in hindsight it's probably better to use a few round-a-bout helper classes (deprecated as they may be) instead of messing with the imported support libraries anyway.
Thanks for this support library project you've created though. It's at least a noble effort.
-Lex