Right off hand, about the only thing I can suggest is try to take the base off the mag, let the spring out, so that the rounds drop free. Then you mighte be able to pull the mag out with pliers. (Put some tape over the jaws of the pliers so you don't damage the mag tube.) I'd use vise grips! An empty mag should be easier to pull out than trying to work with one full of rounds.
If you can find some, vinyl protectant (like Son-of-a-Gun or Armor All) squirted or dripped down between the outside of the mag and the inside of the grip frame (as a lube) might make it easier to remove the mag.
SOG and ArmorAll acts like a lube -- and makes metal rubbing against plastic, or plastic rubbing against plastic slide without as much friction. Auto Parts stores (Advance, O'Reilly, etc.) or hardware stores generally have it.