I just came across a workaround by searching this forum:
I can save a JPG as a DNG and then lock it. ACR adjustments are then saved as an XMP. That would work for my needs.
But my 1st question remains as to whether it can be done with a JPG. Locking a JPG does not work, an EXP file is not created and ACR refuses to save any edits on a Locked JPG.
xmp is embedded in jpegs, so "export settings to xmp" writes them directly to the jpg file.
There should be a way to configure Photoshop to ALWAYS save XMP files, opposed to ever writing them in an image file.
People don't generally expect an input file to be overwritten with a new copy so that settings can be added inside!
Good idea Noel - I'd go along with that .