I looked round a bit but, is there a event for when a player Request the playerlist? IE when pressing Tab?
No, because the server won't be notified. The client doesn't send a packet when opening the list, it only renders information it already has. The server sends ping updates for every player to every player every once in a while (there were huge performance lags because of that, so bukkit removed ping updates completely, dunno how the state is now). So it is impossible to notice that with a plugin without client mods.