I have been in this situation a lot lately, where the leader of a party goes afk for a long period, and then the party has to disband and reform to be able to invite new members. I think there should be a command like this: "/ptcmd mutiny [charactername]"
The specifier "charactername" would be the character to replace the current leader.
How it would work, is if two or more members type the same mutiny command, or choose the mutiny option in party menu and choose the same replacement, then that person is assigned as the new leader.