Short answer: I have no idea...
Longer answer: ... but propane is pretty volatile so I would guess that you'd be ok as long as you gave them an excellent clean and boiled a full batch volume's worth of water for an hour. That should clean it up and build up an oxide layer on the inside. I suspect that any residual propane would boil off and leave under all that heat, and you'd be left with a decent oxide layer to keep your beer off the aluminum.
That is to say, I would use one to make a keggle. I don't think you could actually convert such a thing to be a keg.
I'd wait for someone with more expertise, though, before doing anything.
Holy cow- you guys did it. The Kickstarter was successfully funded! Now the real work
Monticello Barleywine, Red Nosed Tripel
Cali Common, Imperial Common, Sunshine Belgian Rye, Sticke Note Alt
Cherry Blackberry melomel
Belma Pale Ale, Cluster Fug IIPA, American IIPA v1.0, rauchbier, roggenbier