This is indeed, as legandman said, a great idea, and it seems it wouldn't be too hard to do...
But while Armour backpack size variations are a simple matter of model work, the weapon specific backpacks might be harder.
Not that I can think of a way to do it, it just might get a little... hacky, you know?
edit:
BUT WAIT! the flamer is fuelled by cannisters attached to the bottom of the flamer. Now, if your suggesting a HEAVY FLAMER... *Drools* WELL! that's a different story!