Since nobody who knows more has jumped in... the conversion cylinders I have seen have been for .45 Colt, not .45 ACP. When I've seen them in catalogs they say to use cowboy loads only. I understand this to mean the .45 Colt Cowboy-level factory loads or handloads at the same level. But I haven't seen or heard anything to say that the cowboy-pressure .45 Colt factory loads were any problem in conversion cylinders, and much to make me think that's what they WANT you to use.
If you mean .45 ACP when you say that, I haven't seen any conversion cylinders for black powder, cap and ball revolvers that were in .45 ACP. Where I have seen .45 ACP second cylinders they were meant for a .45 Colt cartridge revolver. Ruger, for example, makes a Blackhawk Convertible that includes both .45 Colt and .45 ACP cylinders. In this case the revolver was designed for cartridges from the get-go, and can fire any low-pressure cowboy or standard level factory loading in either of those two chamberings.