One of my favourite charactars from a Japanese serial comic called Gunsmith Cats carries one with a Colt 1903 Vest Pocket in .32. Hers is made much the same way the one from Taxi-driver was made, with the trigger guard cut away on the pistol so when it pops out its right at hand. The guy who draws the comix built one for his Airsofts in Japan, and details the construction...I've got it SOMEWHERE at home. His uses a pull cord attached to the thumb that is invisable on his character because she always wears driving gloves. The spring is a pull instead of a push, and is triggered by pulling down with the wrist, disengaging a break-pin, sending the .32, with the grips attached by a wire to the device, into the hand of the shooter. Not very comfortable for every day use but with a coat or a loose sleeve shirt, for emergency purposes, it'd be a hell of a toy.
It might be a very worthy use for a Raven or a Phoenix Arms weapon- both of which have seemed to increase in quality as of late. My wife brought home a Raven .380 the other night and has put a couple of hundred mixed rounds through it with nary a missfire or feed and with OK accuracy...