i'm back, sort've -- left thumb is pinned and in a cast so it's hard to type.
i dunno what gear i was in, 3 or 4. wasn't lugging at all, really -- and if i were, it still shouldn't lock up the back tire like that. wnet over the bike yesterday and she runs and shifts fine. frame seems ok as well, so i think she's fixable.

:)) (big sigh of relief.)
but i STILL don't know WTF happened.
nothing caught in the chain.
bike shifts through gears while engine running
wheels free and clear
... so i dunno.
otherwise, i can report that i'm not impressed with the "highside" experience and would discourage others from trying it. :P
--Jaeger