I have done some airgun tinkering, but never a muzzleloader.
I would think the place to start is with a hammer actuated valve and the air tank. You need both to function at around 3000 to 4000 psi. Serious pressure.
I am researching a high powered airgun project myself. I've been discussing it with some other builders on the weapons guild forum. One of them threw out the (I think brillient) observation of using a pressure washer actuater valve. The pressure at the handpiece is easily 2500 to 4000psi. If you were to use one with a traditional style side lock as the striker, you could put something together without having to machine the whole valve yourself. You'd also have the advantage of buying replacement seats and springs as you need them.
For my build, I'm going to just use a carbon fiber high pressure tank, or maybe one of the small 'back up' scuba tanks. My gun won't be styled to look traditional so a whiz bang carbon fiber job won't look out of place. You may be able to dress up a scuba or maybe some other industrial tank. Otherwise, you need to fabricate a high pressure vessle. If at all possable, find a tank and make it look right. Maybe use some low temp silver solder to put a couple of brass bands around it to give it a 'Victorian era' look?
Good luck and please keep us posted if you decide to build it. There are some other people working on similar conceps, so look around.
