On the "cowboy side", per the rules, loops for cartridges are legal on the holster and you can use them for your cartridges (I think than you even can have shotgun shell loops on the CAS holster as the style of cartridge is not exatly precised!)
CAS SHB page 11:
Ammunition required for reloads during the course of any stage must be carried on the shooter’s person in a bandoleer, cartridge/shotshell belt loop, pouch, holster, or pocket or be safely staged as required by stage instructions. Leather belt slide ammo loops are acceptable; however, shotgun shell slides may not be worn over shotgun loops on an ammo belt. Shotgun loops must be in a single row. Rifle and revolver ammunition may not be carried in a shotshell loop. No ammunition may be carried in the mouth, ears, nose, cleavage, or any other bodily orifice.
WBAS SHB page 14:
Magazine pouches must be worn on the opposite side of the body from the handgun.
nothing says if the pouches has to be empty or not the rule only says "pouches must be" not "pouches with a magazine in must be"
Such a pouch on a holster is evidently a magazine pouch not a lighter pouch!