WOW, I've been chasing my tail, and apparently for not.

The MPASM that comes with MPLAB 8.73a was generating incorrect opcodes for the banked instructions.
This was making me think things were happening in the chip that should not have been happening.

The actual silicon in the xxJ50's is not the problem.

There is a new version MPLAB 8.76 which has fixed the errors.
Compile your USB programs using the new assembler, and walla, working USB.

There are other chips that will be affected by the problem, and it's not only USB that will be affected, so it is recommended that everyone with PBP 2.60 or later upgrade MPLAB to a version higher than 8.73a.