I really doubt that a solid copper bullet will shoot well in any muzzleloader regardless of the rifling twist unless some sort of patch is used or the bullet is pre-rifled to match the bore and rifling grooves.
In order to load a muzzleloading rifle, the bullet must be smaller than the bore. That makes it much smaller than the rifling grooves.
While a soft lead will bump up in diameter when the gun is fired, I think it is unlikely that a solid copper slug will grow in diameter. If the bullet doesn't bump up in size, there is no way it can lock into the rifling grooves so in effect, you will be shooting a smoothbore with a solid copper bullet. Needless to say, if the bullet doesn't get any spin on it, it will be very unstable and it will tumble or "keyhole".
Paper patching might work with a copper bullet, depending on the depth of the grooves but I think the only bullets that can really be successfully fired in a muzzleloader will need to be soft lead or use a sabot to be work.
Copper plating might work with a soft lead bullet but I can't see that it would provide any benefit. It wouldn't meet any "non-lead" requirement a state may have and if anything, the copper plating would just rub off and contaminate the bore.