I've been given a neglected Spanish made, CVA 'Wolf' percussion rifle (side-hammer version, not an in-line). Whoever had it previously didn't bother to clean it after shooting it. Am I correct in assuming these were rifled with rate-of-twist for (patched) round ball?

You are correct ! I have one and it was butchered by an idiot trying to take the breech plug out , he did not take the drum out first ! used a pipe wrench on it riped the tang off ! I made a new breech plug and tang cleaned the rusted barrel draw filed the damages gun looked new ! I asked $42.00 for the repairs , he told me that I was a crook and to shove it where the sun don't shine ! I still have the rifle trying to find that shady spot to shove it ! Use .440 round balls !