Hi, Tom!
As a prop-loving guy with a uni. degree in organic chemistry, I couldn't sit idly by and see you suffer. So I'm going to try to help you out here.
I do understand your concern, but using a nail to puncture the canister will work totally fine.
The formula for butane combustion is 2 CH3CH2CH2CH3 + 13 O2 -> 8 CO2 + 10 H2O, note that's a lot of oxygen, O2, compared to butane, CH3..etc. Since the gas inside will not be able mix with enough oxygen that quickly, it can't ignite until well after you've punctured it. This will be safe as long as you dont have any open flames (why would you?) right next to the container. To use chemistry-terms, the stoichiometry is not suitable for igniting the gas at the moment the nail punctures the canister.
BUT BE ADVISED, DO THIS OUTDOORS!!!!. If not, the gas won't dissipate fast enough for this to be deemed safe. It could fill up a closed room and make the atmosphere combustive. Which could be really really bad... I'm guessing I don't have to tell you that.
If you're still not comfortable doing this, please don't, and find another way. This was my 2 c on the matter. Good luck!
Side note:
When I was in high school we sometimes took gas containers and emptied them out from a ladder to watch the butane gas/liquid evaporate and dissipate before it hit the ground. Pretty awesome and quite safe. There's simply not enough air for the gas to ignite when the can is punctured, and there is no source of ignition the short moment when the mixture is right.
