Okay, I have thought about this for some time and think that this might work:
Take a plastic .410 shotshell hull (empty) and remove the primer, leaving an opening. Cut the empty hull down in length to accommodate a lead pellet (shot) load. Using a standard plastic .410 wad, insert into the empty hull as one would normally for reloading. Then the lead shot. Fold the plastic top down, just like a reloaded shell (this is the part that I am uncertain due to more pressure to open the fold?). I suppose one could insert a cardboard cap, stopping the shot from falling out.
Then using the now open primer hole in the bottom of the brass hull, fit a probe into that opening with an O-Ring at the bottom to stop leakage/backing out pressure.
The breech end of the barrel will of course need to be milled to the .410 shotshell hull brass head, but that should be easy for any kind of machinist. Using old empty shotshell hulls, would eliminate the need to reinvent something that already exists. All you are doing is trimming the length to fit the need for shot.
Normal shot load is .5 ounce by the way, so with the wad perhaps a total of ± .625 ounces or ± 274 grains. Too much?
Just an idea floating around in my head as I reload shotshells and think that this has some merit?