I would agree that such a device would be legal, unless your state has a law like "switch-derringers are prohibited". Nothing like that in CO.
I wouldn't go to such trouble for a 1 or 2-shot Derringer. Now if I could have something that would instantly whip my G21 into my hand...yes! I recall such a device, called a power-holster, was used by Harry Harrison in the 1960 SF novel Deathworld.