I suspect that Luke's puzzling over "where you were going with this" was because there is no other obvious place to mount any counterweight on the rear of a tractor. If not the 3pt hitch then where ? If your 3pt hitch won't stand the counterweight, nothing else back there is going to. Of course size matters (and each person's situation and job to be done differs) but on an 81 horse tractor weighing around 9,000lbs with loader, etc. I have a 1400lb 7ft bush hog that stays on the rear most of the time. Seems to serve extremely well for a counterweight. And I'm definitely not worrying about the hydraulics handling it for the practical life of the tractor. To answer your question, I think it is fully OK.
With some very heavy 3pt mount items (like a 2 ton boom cutter) it is very advisable to make a support structure that allows the load to rest on the tractor frame and tow bar. Initially the load is on the 3pt but after the hydraulics bleed off the load is on the support structure. I don't think we are into that sort of overkill for a normal counterweight, however.