Some mentors might go on wikivacation from time to time, and would not respond to questions in a given time. It would be helpful to allow them to self-identify as away, so the Homepage will know they're not available at this moment.
This should cause the homepage to not assign this mentor to any new users, and for users that already has this mentor assigned, to display something like "Your mentor is currently not available." and display the mentor with a grey background or something. Ideally, this would also select a replacement mentor, so the user can still ask a mentor if they want to :).