At long last, it's taking shape...
EDIT: 1 September 2015, 2pm
Mwahahaahahaha! Mwahahahaahaha! Mwahahaahahaha! It woooorrrrkkkkssss!
(Caveats: This is is just the "simple" mode, and there are still many rough edges that have yet to be polished out. In particular, be warned that this is just operating on the pose-matrix/head/tail combo, and is not getting applied back out to the pre-parenting+constraints loc/rot/scale values... Solving that problem will still be a significant and hairy challenge, considering that this doesn't work well yet even in other places where similar stuff is done. That said, it's great to finally see this dream taking shape.)