RE: using larger refillable propane tanks:
Cdn Tire (and other places, I'm sure) sell a small brass adapter that connects to a refillable tank. The other end has the same connector as is on top of a disposable tank. Take one of these (buy the double outlet one, it costs the same as a single, and gives the nozzle better orientation) and connect a metal or AI fill adapter. With the tank sitting on the ground, the nozzle points straight up. Pick the tank up, turn it over, and you have the world's biggest fill can!
i recommend using a 5 or 7 lb tank, as one of those can be handled by a single person. OTOH, if you want to establish a filling station for a two hundred person team all using GBBR's and you have access to a crane, go with a 100 lb tank... ;->
I used the above setup for the past two years with a 7 lb tank. Works like a dream, gives great fills. Not really portable like a can of green gas though. Solved the problem with one of those cylinder re-fill rigs... looks like it will work well.
Why work so hard for cheap propane? Well, for one thing I like gas guns. For another thing, I HAVE worked out a way to load silicone oil into an empty re-fillable tank, so I actually have 7 lb of green gas, not just propane.
Note on safety systems in propane tanks: it seems that they will vent liquid if fully inverted: at least my 7lb will, and it's only 2 yrs old. IIRC, it will not vent anything if it's laying on it's side. I also have a tank on our tent trailer that will not vent anything at all unless it has a coupler hooked up to it.