We did have one a few years ago where Cap Prem players for one club turned out for a Cap 9 team at another club.
It was protested but primarily because the game was on Wakefield 2 next to a big Chatham Cup game and the player's in question were spotted by their normal team mates who worked for Capital football.
Their anonimity also wasn't ideal considering the father of one of them is an ex-Nighthawk.