quick idea I had tonight for a self releasing magazine. Not 100% sure it'd be possible, but basically it's a small spring behind the magazine follower pin, so that it can be pushed back into the body of the follower. have a mechanism with a slight angle to the bottom that would push the pin into the body when the magazine is inserted into the magwell.
Here's a quick picture:
Jog in mechanism is to get around screw boss. spring on the bottom to return it to the 'ready' position. Just load up the rounds to push the follower down and you'd be good to go. Follower is held down until insertion by one of the ribs in the magazine until the pin is pushed into the follower body.