I have seen X ray pictures of breech plugs from high quality English long range rifles that were obviously drilled from the left to the right all the way across and then theaded and a plug installed,dresssed of,polished and engraved and it took X ray to find it. On the bolster side,the threads were plugged with a similar screw and then a platinum plug installed over it. This gave rise to the idea of platinum "blow out"plugs which they were not,The platinum or sometimes silver was only ornamental.
The pictures I saw belonged to the late Don Brown who marketed Alex Henry replicas and components to make them and the breech plugs were from some original 1860's vintage rifles that he owned. This was in Canada in 1987 at an International match.
If the material in that pistol plug is weldable,do it,file it,polish it and then go to the next step on this project. I have never heard of anyone trying to remove this type of plug and most current owners of these guns don't even know it exists.I started in muzzle loading in 1951 and first knew of it in 1987 when Don showed us the X rays.
Bob Roller