TwiceBoom
I,too, have this idea. To share some thoughts. I have an original sxs 12 ga. percussion with the type of breech plugs you describe that is a hooked breech with a hook coming off each plug. TOW shows hooked breech plugs which consist of only the threaded plug w/hook coming off it for use w/Hawkin style rifles. This would place the butt of the barrel directly against the tang portion of the breech.
If the two plugs are threaded w/ a stub of shaft on the end, the hooks can be hacksawed & filed to shape after they are screwed into the barrels. Cutting & shaping the tang part might be interesting, but also doable w/mostly hand tools
That is my theory, anyway. If it really isn't feasable, I'm sure someone will come along and shoot me down. Or, with the expertise on this forum, maybe even fill in some of the holes.
Paul