The back purge is a good idea on bicycles because the steel is so thin, and there is a high likely-hood of melting the back side of the tube. Are there any problems drilling copper? I'll likely end up drilling a 1/2" through-hole that is 4-5" long
I'll probably end up purchasing this from metal supermarkets and the price for copper is only about 15% more it looks like. I might end up needing 4 or 5 of these so I'll price it out
Drilling can range from easy to miserable, depending on, it seems, the phase of the moon and the way you hold your lips at the time... Seriously, never had the same experience twice. Have strong suspicion that how work hardened (from being rolled to shape at the mill), or how annealed (from exposure to high heat) the bar stock was.
Tapping the stuff was absolute hell, up until we used animal fat as a lube. Try butter, or dip the tap in high fat coffee cream. Bacon fat will likely work at least as well. Real lard too, if you can get it!
Did a project that required several hundred tapped holes on a grounding strap 1/4 inch x 2 inch, by 6 feet long, in an air traffic control tower radio room.
For simple back purge fittings, I suggest making some plugs from aluminum, drilled and tapped for gas fittings, and with a couple O-rings to seal around the inside surface of the tube. Cheap as chips to make, and you can make them as required, depending on your tubing sizes used. Just an idea. Less dicking about than dealing with aluminum tape.