I've added a new field to the prospect tab titled "Endowment Stage". It is searchable and uses the same drop down selection as the MG Stage field. Also, I renamed "Stage" to "MG Stage".
For reference I also added this field to the summary screen for MG users.
Eileen noted "we won’t have logging on this field until we update the triggers but I’m guessing that will be pretty secondary to you". :)