This would be an instant dead gamemode, as one of the main reasons people go to private servers is for not having a membership fee to play the full game, yea it might have players if a tournament is made, but im sure it wouldnt survive longer.
Support its a dumb feature osrs dosen't have it, if you want a 1defence account later on make a new acc its easy!
Also a big fan of those communities you can lock xp so this should defo be done