For WBTS reenacting, it was a standard safety rule amoung Federal and Confederate Reenactors that nothing but powder was/is allowed down the barrels since the early 1980's for sure and may or probably before that. I know it was that way then for folks doing living histories on National Park Property.
When I first began doing WBTS reenacting in 1980, they still allowed us to throw the paper cartridge down the bore AS LONG AS there was no staple or anything solid on or with it. Then a year or two later during a Tactical or Civil War period "War Game" the day before the reenactment of Balls Bluff in Southern Maryland, one of the smoldering cartridge papers was shot out of a musket and set the super dry grass on fire. We stopped the tactical and many of us ran to stamp out the fire and used blankets to put the fire out before it got out of control. Then we went back to the tactical. The next day no one was allowed to have anything but powder in their bores during the reenactment and the dry grass was not set on fire.
After that incident, NO ONE was allowed to put anything down the bore except for powder at least here in the East, from the Carolinas up through at least Pennsylvania!!!
The "Nine Count Loading Manual" called for loading the barrel before priming with a cap. So that's the way we did it with blank rounds. The chances of the powder going off in the bore were extremely thin and though the word "rare" is way overused, it was really a rare incident when the powder was set off by embers in the barrel. Angling the bore away from one's face and being careful to pour powder from the paper cartridge at an angle to the bore ensured the powder in the blank cartridge was never set off while the reenactor's hands were near the barrel. The very few times the powder cooked off in the barrel happened in 8 years of very active reenacting here in the East, there was never a case where the powder was ignited until after the paper cartridge was discarded and the man's hand was away from the muzzle.
The funny thing for me was the worst thing that happened to me with a Brown Bess was during a "Show Battle Reenactment" outside the National Archives and on Pennsylvania avenue on the 4th of July in the early 2,000's. The temperature was close to 100 degrees and the asphalt on Pennsylvania Avenue we were standing/fighting on was between 110 and 120 degrees. They allowed us to do "The Highland Drill" or Highland Skirmish drill. That meant we advanced in small groups, fired at will and knelt down on one knee to reload.
HOWEVER, the first time I put my knee that was not covered by kilt or bag hose - down on the hot pavement, I did NOT do that again. GRIN. After that I just crouched. The pavement was HOT even through the soles of our period shoes.
As we loaded and fired very quickly, the barrels heated up so much that it burnt the skin off our fingers and hands and stuck to the barrel. It was a real PITA to clean that burnt skin off my barrel in the 6 or 7 places I "left" my burnt skin on it. Grin.
Gus