I think this will be part of the current refactoring of connection.
With that, you will be able to set a device address with a relay, if Iβm not mistaken.
Itβs actually somewhat [documented](file:///Users/jb/src/github.com/syncthing/docs/_build/html/users/config.html#options-element): the syntax is relay://host:port/?id=... and so on.