For a true survival muzzleloader I would choose a large caliber flintlock musket, like the .75 caliber brown bess flintlock.
If you are unable to get .715 roundballs, it will shoot smaller caliber roundballs, shot, small stones, coins, nails or even rock salt if your life depended on it. (might scar the barrel some, but this is a survival scenario, right?)
Anthing that will fit down the tube (in theroy) can be shot out, including wooden slugs made from a dowel rod, a heavily patched nut and bolt, ball bearings, marbles, BB's. The list is endless, may not be good for your barrel but it could save you from being killed.
Most flexible material found in nature can me made into wads, bee's nest, leaves, straw and grasses can all be packed down a barrel to work. Even a US penny or dime will fit into the tube, could be made into a wad if it was a life or death situation.
Once you are out of percussion caps, you are done shooting, but if I run out of flint, I can use any sparking stone found in nature with a little work, even old Indian arrow points broken to fit in the jaws will work (held sideways using their cutting edge).
Pyrite (fools gold) is another choice for sparks, even hard sharpening stones for knifes will spark once knapped a little and broken down to fit into the jaws.
Note: I am not saying to use flint arrow heads, coins or nails in your guns, but if I was in a life or death situation and thats all I had to work with, I would not hesitate to improvise.