I think you gents may be dipping into the wishing well a little to deeply. I seriously doubt there is enough metal in a GP100 cylinder to fit even 5 rounds of 45 Colt in it. Note that even a S&W 625 is an N frame sized gun, and it only shoots 45 ACP.
Even if you could put them in, the cylinder walls will be scary thin. Ruger doesn't have a reputation for underbuilding their guns.
Take a look at images of a S&W 696. They were L frames chambered in 44 spl. The cylinder walls, and the forcing cone for that matter were really thin. Granted, I believe the GP cylinder is a bit thicker, but not enough to matter. Take a look at the 44 spl GP100 also. All someone would have to do is put a "Ruger only" load in a 45 Colt GP100, and I'm guessing that gun is going to grenade after a few rounds.
Ruger likes a wide safety margin, as is apparent in their designs. The concept is wonderful, but not likely to ever come into being. If I were a smith, I wouldn't even attempt it for the potential disaster that could happen.