Left hand twist drill bit:
www.kbctools.com
There are some left hand cutting tools at Home Depot. I don't know if they go down to a small screw like a mag release screw though. If you've got a dremel, you can cut a slot in it with an abrasive disk and crank it out with a screwdriver. You can also get brutish and drill out the screw with a 1/8" bit like ClotFarmer says. The head will drill off and you can remove the button. With the head removed you should be able to turn out the threaded stem as they'res no tension on the thread (unless it's loctited in place).