We'll think about a menthoring system, and if you guys have any good idea, post away please.
What about a "mentor token" that can (somehow) be created by a new player, and given to their higher-levelled friend. Then that token can be given to a de-leveller NPC, and their level adjusted accordingly. However, in order to prohibit exploitation of such a mechanic, the mentor token delevelling only lasts for X amount of time, before the player is returned to his/her original level. That way both sides compromise, higher levelled players can mentor their friends, and it doesn't last forever, and abuse of the system hopefully won't occur.
Mind you, I've only had 3 cups of coffee so far, the above suggestion may be rather incoherent.