from what i see the threads simply provide a method to tighten the upper cup. without them something would be required. if that something is simply a top cap and expanding carbon steerer plug (or just your hand), i don't see why it wouldn't work. the reamed cup would need to be a tight sliding fit.

i don't see why anything like a compression ring would be necessary. the stem will be in contact with the upper adjustable cup, which in turn will be in contact with the bearings just as it should.

in essence you will be using the top cap (or hand pressure) to provide temporary security and adjustment to the headset during assembly and the pinch bolts on the stem to maintain it after assembly rather than relying on the threads on the adjustable upper cup and steerer tube and a lock nut.

IMO, the idea is good enough to take the time and effort to give it a try. good luck. if you decide to do it, let us know. i, for one, would be interested in the outcome.

BTW, i would think that as soon as the adjustable cup slides over the steerer tube the reaming can cease.

OTOH, i don't know how many times i've sat down to do something slightly unusual on my bike, thinking "this should be easy" and a couple of frustrating hours later thought, "dammit! this is not going to work at all", but this caveat may be unnecessary, for i think you may be a member of the choir.