Steve H, I know of no .22 Magnum semi-auto "mouse guns". True, there are the small NAA revolvers and some snubbie .22 Magnum revolvers, but I have never heard of the creature mentioned in your original post. My gut feeling is manufacturers believe the cost of developing such a pistol would not justify its costs. Please do not misunderstand me. I am not opposed to the idea of a maggie mouse gun, but I do not see any incentative to build one. I hope this helps.
Timthinker