Well I don't know what others have done, but, I have syphoned many a spray can to get at the liquid inside, which I promptly thinned for airbrushing.
Met a Master Chief at the BUD's quarterdeck one time who turned me on to the technique. My buddy was actually buying an original "Rifleman's Jacket" from him but we got to talking about all sorts of shit. Anyways, what they did was go to the local Kawasaki shop and buy cases of "Kal Guard" (the predecessor to KG Gun Kote). They coated their CAR-15's and MAC-10's inside and out with the stuff, to increase corrosion protection in a maritime environment.
We went down to the local Kawi shop, and what do you know, they still sold the stuff. So we scarfed a bunch up and began to experiment. We spray it into "film cans", using the hoses from "Brake Parts Cleaner" cans. Luckily for us (we figured) it was so thick, we had to thin it down to get it to spray through our hobby airbrushes. It was this beautiful charcoal gray color; looked like a really nice park job. Back in the early 80's, when you built an AR, it was all sorts of shades from all sorts of different mfg's. So the "Kal Guard" coating really gave you a nice factory finish.
I don't know for sure, but if I wanted Alumahyde (which is good stuff) I'd use this same technique to run it through my current spray rig.
And I just buy liquid direct from Dura Coat.