I used "dreamx_follower options" plugin to include guest characters. The guest character is an actor marked with notetags and doesn't show up in battles (the notetags can be set to a switch, so a guest character can become a regular if you want), but his/her stats still appear on the menu screen as well as their equipment. I didn't care much for this detail, though. Perhaps there is a workaround with another plugin.
Oh, and according to the patch notes it is compatible with Hime's Custom Party Leader plugin.