People do make reproduction flintlocks, and, given that the majority of these are basically custom order…

… you could probably pay for the barrel to be threaded, to allow for it to attach a supressor, or possibly just ask the gunsmith to make a custom, integrally suppressed barrel.

Problem with the threaded barrel is that most flintlocks fire very large caliber rounds, so… finding an actual compatible and attachable suppressor would be difficult.

… it would be a jokey gimmick gun, but you could do it.