From my experience the straight fluted easy outs
http://www.google.com.au/url?sa=t&r...sg=AFQjCNGLuZuGyXHXZXGEQNMSBvLBRBZyxQ&cad=rja are much more effective than the spiral fluted. They are essentially a tapered square profile with sharp edges. You hammer them into the hole rather than screwing them in.
The other thing that helps, though they can be hard to source is using left handed drill bits
Amazon.com: Irwin 30520 5 Piece Left Hand Cobalt 5/64-Inch to 19/64-Inch Cobalt Twist Drill Bit Assortment: Home Improvement. (this isn't like the old striped paint trick to play on apprentices.) They are normal drill bits that cut when being spun anti clockwise. Often the combination of the drill bit removing the metal (thus relieving the pressure on the threads) and the force being applied in the correct direction (to unscrew the plug, rather than screw it tighter) is enough to get it moving.
My suggestion would be trying to hammer a larger allen key/ torx bit into the plug and hope it spins. You'll get better control if you use a small spanner on the shaft of the allen key right next to the face of the plug, while pushing down hard from above.
Then I'd drill out the plug with the left handed drill bits (if available), and get a small cold chisel and try and hammer it loose with that. Sometimes drilling it out is enough to relieve the pressure on the plug.
Then I'd try the easy outs. If they don't work drill it all the way out until only the threads remain. Then try and pick them out with a sharp pick. The only option after that is putting a lot of heat into the reg body* to try to get it to expand, and finally you can drill out the whole plug and rethread it using Helicoils*
http://www.google.com.au/url?sa=t&r...sg=AFQjCNGM5TxA5YEyvnHEqAjtD_Qs53CZQA&cad=rja. Though this might not be advisable on a scuba reg.
One important thing. Don't drill the first hole so big that the easy out projects too far into the body of the reg. If the easyout bottoms out on the reg body inside without gripping the wall of the plug you'll need to re drill and go up a size in the easyouts. Patience is a virtue with this kind of thing!
All of this info is based on my experience as a mechanic, it is not specific to scuba regs. I haven't got any direct experience working on regs... but I have removed a lot of broken bolts over the years. *this will pretty much destroy the reg I would think.
Finally, if you snap the easyout off in the hole... just walk away.