Craig,
It sounds like you are all set now (are you?).
If you need to "correct" existing files, perhaps a text processor instead of exiftool, e.g. a grep-like utility, or text-editor...
Now that I think about it, that can be problematic since APP tags have a length field that must be attended to.
But here's a 3-step solution:
1. Extract xmp from jpegs using exiftool.
2. Correct xmp as mentioned above (strip JRMC stuff).
3. Shoe-horn the modified xmp back into the jpegs using exiftool.
?
Rob