I'm not sure if this has been mentioned, but the mod "Set-up camp" (http://steamcommunity.com/sharedfiles/filedetails/?id=932951772) uses an interesting system for setting up temporary camps with caravans. I have absolutely no knowledge of modding, but the edges of the map have some sort of trigger that forms the caravan back up when you draft your colonist and send them to the edge. Maybe this function could be used to transfer between "overworld" and "tunnels", where you have the trigger tiles placed on some kind of buildable lift transferring pawns from one map to the other.