The way around the antique pin-fire conundrum is not to actually own it. Give it the wife and then borrow it when you want to use it. So long as you have your shotgun certificate you are covered and so long as you hand it back within the 72 hour deadline. I've put that question to one or two and no one can see anything illegal in the exercise. Might be worth running it past BASC and see what their take is on the idea.
Making pin-fire cartridges is covered in Geoffrey Boothroyd's book The Shotgun. History and Development pages 181-3. The idea was from a late friend Derek Fearn of Catton Gunsmiths. Access to a lathe to make the percussion cap inserts which then fit into a regular plastic cartridge case. He used to use compression formed Winchester cases with the heads peeled off. Lagopus.....
