If you want to go to the trouble of machining a frame for a pistol, by hand or otherwise, you can get as cast, or partially machined frames from places like Sarco. Or pick up a copy of Shotgun News and browse through it for others.
And, by the way, 26" OAL IS the limit. My mistake.
All the regulations can sometimes be minimised by going with a "non metallic cartridge " design, but there is a caveat to federal law. It is against federal to violate state law. Same usually goes for local law and state law. That's where state premptions come in handy.