Nim,
When shooting sabots the caliber of the bullet is less important than the fit of the sabot. A snug fitting sabot that is not too hard to push down will typically be more accurate than a looser fitting one that goes down easy, but not always. You can get sabots for both 50 and 54 cal guns that will work with most common smaller caliber bullets from .400 to .500(for the 54). The most common sizes will hold .429 or .451 bullets and there are a lot of bullet choices in both of those sizes. Since you are limited to non lead bullets and Barnes is the most common maker of all copper bullets you should look at what they offer. I have shot the 285gr Barnes in a couple of my 50 cal guns and they work very well, the 250's also shoot well. Give them a try and see what works best in your gun.
As for the 45cal, I don't have one but I imagine someone probably makes a copper bullet that is under .451 that could be used in a sabot, I just don't know who or what size. I do know there are 45cal sabots for a 40cal bullet available so if you can find a 40cal copper bullet you can buy the sabots in bulk.
Good luck in your search and don't worry about saying "traditional" on this forum, most guys over here are willing to discuss anything that relates to MLing.