I've been mulling an idea for a while. Take a strong .22 s/l/lr BA action with a threaded receiver and mate it to a BP ML barrel. The barrel breech would be chambered for .27 RF blanks. Instead of using 209 primers by hand or using caps, simply work the action and load blanks into the breech to act as primers. Perhaps a variation could be a magazine fed 209 primer version? Thoughts?

Yes you could. I was making muzzle loaders on bolt actions, way before the modern inlines were invented. I used musket caps, and 209s. Game wardens would ask me about them because they looked like Mauser sporters. Even did a couple on P14 enfields. You could even make one using primed brass as long as it couldn't take and fire a bullet.

Some BP substitutes (Triple Seven ?) perform best with low power primers intended for the 410. Others work best with the hottest 209 primers.

A while back, someone was selling a breech plug conversion to allow use of either 25 or 32 pistol cases with small rifle? primers.

The same people who think pellets are a good idea would probably love magazine fed primers. Colorado will ban it, like pellets.

Someone will get excited and chamber a fresh primer before loading the barrel. Yes, that can be done now, but it takes a little bit of deliberation.

I vote: No

If I was king, I would not even allow cap locks; way too modern. Modern MLs are already so far removed from the spirit of "primitive weapons" that it is laughable.
Not germane here though.

The only BP primer the world needs if FFFFG!

Yes and actually, in the Military Musket era, they were issued cartridges containing "musket powder"; they primed first, then poured the rest down the barrel.
Sorry Bob; go for your idea; it will work. Someone has derailed this thread.