My take:
Mentorship is, by its nature, an OOC thing. This should be acknowledged and respected by the game mechanics that support it. Yes, it's cool to be able to roleplay a mentor-student relationship in-character, but that is and should be secondary to helping people learn to play the game.
Because of this, mentorship should never be bound to any in-character construct such as a realm.
Forcing new players to look to the forum for mentorship is tantamount to abandoning casually interested new players. They will not be willing to take that extra effort.
Thus: It must be in-game, but not in-character, and not realm-centric.
My primary impulse is to just create an island-wide "all mentors and students" channel, possibly limited to out-of-character messages, and prevent contacts from being created between two mentors by messages sent through the channel. Then mentors will easily be able to communicate with all students, and through personal replies, to their own students, but there will no longer be a specific tie between a particular mentor and student.
