If one hasn't actually used 777 in a sidelock I'd suggest one might consider trying it before telling others not to use it.
777 will work just fine in the Pedersoli Kentucky caplock rifle. Yes, many inline shooters prefer it, but that's no reason to avoid it in a caplock. It does have a higher ignition temperature than real black powder (so, by the way, does Pyrodex) but that's not an issue for percussion caps. Flintlocks are a different story.
Go ahead and use up the 777. Use 15% by volume less than you would with real black powder, and don't use heavy compression when loading and you'll be all right.