Attachment 189447
Am I imagining that the slot isn't quite central or straight? It isn't much worse than the takedown system of the Greener GP action, which people have successfully built into rifles which work with modern .45-70 smokeless loads. The only real difference is that the screw on this Ballard isn't as far from the barrel, and will therefore exert less leverage in tightening up. If the receiver limitations don't put you worse off than the breechblock limitations, what difference does it make?
If I wanted to put it back to solid-bridge, I would silver solder a piece of sheet steel in the slot, using Brownells 355 silver solder, which comes in thin ribbons. I don't think you would really need to do any special blackening of the solder lines when you have blued it. I have a 24ga shotgun which is in as near mint condition as you dare expect in a 1926 gun, and I think the reason is that an Australian got it home before he realised that 20ga shells wouldn't fit. There is a silver-inlaid "24" at the rear of the barrel rib, but it had tarnished to be indistinguishable through ordinary atmosphere and handling.