I tried .570's in my caywood trade gun. .570 ball, and two .017 patches. the patches were still really loose, and my groups from a bench were not very good even at 25yds. I think probably thicker patching would have helped, but I didnt have a combination with me that worked good. Like you, I do like the idea of using a more common ball if it will shoot well. I will try it again, but was discouraged so far.
I got very good groups at 25 and 50 yards, with .595 ball, and .010 patch lubed with bore butter, but .595 is a pretty odd size. I just happend to find 100 on consignment, and they worked awesome, but I have no good way to get more. I think TOW has them, but way too expensive for me to justify. Might have to get a mold for .595 I guess, but I would rather figure out a good way to shoot a more common ball. .60 and .61's are too big, cant patch them, and I get bad groups shooting them naked. (plus the neighbors get nervous)
Good luck, Jeff