Well, AFAIK the only "standard" applied to Cowboy loads is that SASS officially recognizes such a cartridge as having a muzzle velocity below 1000 fps.
If that's true, there's alot of such cartridges in the world.
I won't be using +P loads at all. If anything, I'll be using smokeless in very small charges only where "needed". I'm no fool that thinks smokeless and BP can be interchanged volume for volume, or anywhere close. I'll do my homework.
My repro Remington is a steel-frame. I do know of atleast one person who loads cartridges in a conversion for his brass-frame, but those are very light loads he puts through.