There needs to be no barrier to entry, but no "reward" either. The system should be you wanna be a mentor you can be a mentor. /mentor turns it on.
That way the flag, whatever it is, isn't "exclusive" since you can turn it on at any time.

There should also be a limitation of if you have so many substantiated actions against you, you cannot be a mentor. Because that's basically what a mentor should be, someone willing to help and not be abusive, for the sake of being helpful. Unpaid community management.