I don't know of anything commercially, but couldn't one take the collapsible buffer tub, fill in the holes "ahead" of where you want to stop with JBWeld, and then fill in the "groove" behind where you want to stop?
For example, say you want to stop in the 3rd hole of 6. Fill in the first two holes with JBWeld so they're flush with the deepened groove. Then fill in the deepened groove and holes 4, 5, and 6 behind hole three, flush with the circumference of the buffer tube. The stock would stop automatically only in hole 3 but would still be removable if you wanted.
I don't know how JBWeld holds up to firing forces over the long run, but it's worth a try.