The main reason I reload black powder shells only once is I don't feel like cleaning the powder fouling out of them and I use the cheap shells that can be picked up by the hundreds.
I'll re-reload them if I'm going to shoot them immediatley.
At muzzleloading trap matches, instead of taking a bunch of reloaded shells with me to shoot the black powder cartridge class, I take a portable reloading press and a bunch of components. I tend to shoot three shots at one station and then pick up those shells and re-reload them and then shoot three at station 2, etc, shooting the entire 15 bird match with the same three shells.
The sweet load for my 12 gauge muzzleloader is also 80 grains FFg and 1 1/8 ounce of shot, except I use two .125 nitro cards instead of a nitro card + fiber wad. The main reason I use fiber wads in the shells is because the total length of the load column has to be a certain length in order to get a nice crimp.