Sgt. D, you make a very good point - especially so that a competitor could "mix'n'match" firearms at the same shoot - and such a rule would hopefully prevent the Class from turning into an "arms race" worse than anything SASS has experienced!
("Gotta get myself the the best, quickest repeating rifle I can manage to justify within the allowed timeframe!") Mind you, I suppose that it may be impossible to c
ompletely eliminate all elements of such competitiveness/gamesmanship. I guess (as Maj. Matt intimated) creation of such a shooting class will definitely require considerable thinking out of the various implications .... I know where you are coming from with your hope of avoiding a subdivision of the class into single shot and repeater, but (to play the Devil's Advocate a bit, here) perhaps such a subdivision would be unavoidable, when all considerations are taken into account ....
On the "downloading issue" - As you say, the Snider round is relatively simple to load down, using wads or the like. Another method which has been used to reduce effective case capacity is inserting a thickish tube/collar (cardboard or whatever) inside the case body, coming up to the point that the bullet base is seated to. This method can also work with the M-H cartridge, though it is a bit more "fiddly" to accomplish.
Actually, a very good method of downloading these big-bore BP cartridges has been developed, which works very effectively in the bottlenecks such as the .577/.450 while avoiding the danger of a "plug" forming in the neck if an inert filler like Cream of Wheat is used. That is to mix equal volumes of black powder (or substitute powder being used) with your Cream of Wheat (or whatever), and use that as a filler above even much-reduced powder charges in the base of the cartridge. The filler is then not "inert", but rather the propellant mixed with it combusts along with the main powder charge, preventing the filler from compacting .... One does have to factor into one's charge calculations the amount of powder mixed with the filler, however. There has been a lot of discussion about this method, and the success achieved with it, over the past few years on the "British Militaria Forums" - where a bunch of Snider- and Martini-shooting maniacs like me hang out ...
http://p223.ezboard.com/bbritishmilitariaforums (Matter of fact, the Martini-Henry rounds I shot in the Long-Range Big Bore Single Shot rifle event at Muster this year were somewhat reduced loads using this method - total propellant equivalent of about 60 grns of powder, rather than the full service load of 85 grns.)