that would work if I used a nut the same size or bigger than the trunion threads. every thing else would be the same as you described. except the smaller nut would still go against the trunion. it just goes on from the front and stays with the barrel with the Gb installed. this will you to reduce the profile and also use all the take off barrels out there that can be rechambered.I guess I'm missing something. The way I'm envisioning it I'd thread the nut onto the barrel at the breech end. Turn it on, advancing it toward the muzzle until it contacts the thread shoulder.
Turn the barrel/nut combo into the threaded trunnion, advance/retreat until headspace is set, then tighten the nut against the trunnion. Re-check headspace and if still correct, make witness marks on trunnion and barrel for future headspacing and retiming.
here some picks how it looks on a a trunion with a nut it still locks on the trunion face just the nut threads are smaller ID to take advantage of the profile reducing on many barrels and to allow a nut to clear the RSB as well allow modifed factry hand guards to fit.
got to think out side the box guys HMMMMM a OTB47 swithbarrel rifle would be a good name