but to disallow editing non-pp ICM file data seems absurd.
It does not "disallow", it just isn't designed to do it.
I wonder if the software supplied with equipment such as Xrite i1 Photo will edit profiles made with a equipment from a different manufacturer; I somehow doubt it.
One suggestion, if you use Qimage Ultimate for printing, it's possible to use a Print Filter to correct consistent colour errors in prints. This does not alter the image in any way and applies the correction for printing only.
Terry