We thought long and hard about this when we started planning the Musters. The NSSA has a musket category, but their competitions are quite different than the GAF.
First, as noted by Dusty, it takes a long time to reload and shoot, which doesn't make the category very compatible with the cartridge arms.
Second, there doesn't seem to be a great demand for it. If we could come up with enough individuals to make it worthwhile, I'm sure we could also come up with some rules as to how those equipped with muzzleloaders could participate in a Muster. We could always offer it as a side-match, which I believe was done at least once, but again there was only a small turnout for it.
This is not to say that shooting muzzleloaders (ie, muskets) cannot or will not be done at the Grand Muster or the Department Musters, just that we will have to consider how it can best be accomplished should there be sufficient interest in it. If anyone has some recommendations on how we might do this, just send me a PM and I'll have our shooting committee take it under consideration.
I was shooting muzzleloaders long before I ever heard of or starting shooting CAS so I am not against including such a category if there is sufficient interest to justify it.
US Scout
GAF, Commanding