Scope mounting level???
 

What is the reference plane used on a muzzleloader to obtain a level platform for mounting a scope?

On my centerfire rifles I can mount a level in the breech to accomplish this...

Well if you have a gun vise or something to hold the rifle in a fixed position here is what I do. First I mount the bases which gives me my initial platform as you put it. Then attach the scope rings, the scope and the other 1/2 of the rings. Just tighten the screws enough so the scope can move freely with a little assistance. Shoulder the rifle and adjust scope for proper eye relief. Now in lieu of the level-level-level system which uses 2 levels, I simply use a line level to make sure the initial platform is level (scope bases). Once leveled, and being careful not to move the rifle, lay the line level on the top turret cover and level the scope. Gently snug the screws up. Once the screws are sufficiently snug you don't have to worry if the rifle moves and you can tighten up the screws.
Sounds a bit complicated but it actually takes more time to type this than to actually do it. Also, the use ot 2 line levels is even better as you can keep on on the bases and the other on the turret. Now if your base is integral to the rifle, you can attach the lower 1/2 of the rings and use that as your platform. But then you have to be careful you don't move the rifle when putting on the scope. And, you have to know your eye relief before leveling up the rings.