At last, the 'Local' option for Armatures works properly!
Tonight I went through carefully and cross-checked the code once again, and found several bad mistakes I had made. These were:
* the value of one variable from the armatures code was not what I expected it to be, based off the name).
* Mat4MulSerie swaps the first two args! Grrr...
Note:
There's only one rig that I've tested that was broken. That was slikdigit's "mancandy", and the part in question was the jaw. It is likely that a few more rigs out there (in particular, their 'local' action constraints) relied on the wacky rotation values that used to be used, so are now broken.