This trust manager idea seems awesome.
I'd think of a chat message saying "[player] has invited your for [build/full] trust, check the manager.", then you'd have a trust invite manager that would manage those invites.
In that manager it would list all the trust invites, if they get too old, they will automatically be deleted, same if the player leaves or you leave the server. There would be a button "accept" and a button "decline".
I'm too busy with other projects so I'm not going to say I'll make it. Just sharing the ideas.