I was fortunate enough to correspond and trade with Harry O on Leverguns. His articles and your comments, work on Heel bullets, is timeless, and a boon for anyone trying to explore these obsolete but interesting type cartridges. I too really have been interested in the .41. Both my guns I used as platforms were the 1892 and 1895 Colt New Army Navy DA guns. Not a very good firearm actually, a transition piece. The cylinder locking leaves a lot to be desired. Complex lockwork.
Heaven help anyone trying to sort an old Lightning or Thunderer. I don't think there is but a handful of guys in the world left to manage that. Very quickly the cartridge makes you decide if you are going to use heel or hollow base bullets. By far the consensus is the hollow base are easier to manage. But a .386" bullet rattling down a .403" bore is daunting. But it seems to work at least a little. (Brains exploding of modern reloaders.) To think some fret over cylinder throats and sizing down to the exact thousandth. You GOT to do that.
Making a .41 Long Colt gun would be relatively easy. Re-bore a .357 cylinder and fit a 10mm barrel. I never could sort out the making brass from .38 Specials! Mine all split. Harry O was quite bemused at me shooting 10mm truncated and SWC bullets "upside down" out of the .41 Long Colt since I didn't have a proper heel bullet mould. I finally got 3-4" groups at 30 yds. and he said that was quite good! You are right, Colt SAA are astronomical, even .41 Colts. But I might dive back down that hole one day. A GP100 would be a very naughty project, now that Ruger has the 10mm barrels for them. Nothing to be gained by that, just to confound shooting buddies, and have the only one.