If you look at this pic, you can see where I put down a 'bead' of soft solder. That was enough to catch the rim, but of course it eventually wore down.
Final solution was to solder a thin shim (IIRC .003") to that location. PITA to get it all together....
The shim didn't interfere with 10ga. Every so often a 12ga rim would slip over it, but still pull it enough to pull them out with my fingers. No big deal.
When the 10ga group came together and tested the loads, I started to shoot light 10ga loads almost exclusively. (It helped to find several thousand 10ga wads at a pawnshop in Montana.) I think I still have the tubes around here but haven't used them in years. Actually haven't shot the gun in over five years.