I know of several servers on which you can disguise as certain mobs. I've never heard of disguising as players.
Disguising as Players has been disabled. You should get a message from the console saying: "You do not have permission to disguise as the player: <player name>".
Yeah, I was going to say, I've never seen anyone disguised as a player before, so I'm pretty sure that you cannot.
People have never been able to disguise as players on Minetime, nor has it ever been listed as a perk.