I'm not sure about the extende3d cap, but you can get a Fasst Change from PPS, IIRC.
And Palmer has a point--a QC is simply a nipple. A fill nipple is a QC with a sort of check valve in it. Thus, if you keep the nipple there but change how the air goes in, it won't leak.
If you follow that route, have the valve side tapped, put in the fill nipple, and you could switch between 12-grams and constant air of any sort, using a 12-gram as a crappy pressure chamber.