Allow masquerade_menu_access() to be extended via a hook()

Issue Summary

I would like to use masquerade for subuser_switch which uses the relation project to store parent-child relationships. Duplicating the information into masquerade_users seems to be the only way to make the permissions work as they need which is really not a good idea.

Can we add a hook of some sort to allow someone to alter/define their own masquerade access rules (aka subuser_switch only allows parents to switch to their children)?