Capdiver, I am in need of exactly the same thing. I have an older SeaQuest in good condition except the inflator fitting broke at the bladder. I think they must have had an unstable plastic in those models.
The rear dump valve did the same thing, but I was able to find an old ScubaPro valve that fit the 2" hole. Most newer ones are all smaller, as are the inflator elbow/dump valve fittings. If I find a cure I'll pass it along, and please do the same. Hopefully someone on the board has a cure for replacing a shoulder inflator fitting with (for me at least ) a 2" hole.