Mir's default is already $XDG_RUNTIME_DIR/mir_socket. (Also, it's correct that /run/mir_socket is not for apps - it's only for unity8 to connect to u-s-c as a guest (nested) compositor. )
However, $XDG_RUNTIME_DIR cannot be used for the u-s-c, as it'd be against the XDG spec to have rw permissions for non-root on the directory, so phablet user (which unity8 runs as) would not be able to access it. I am guessing that's why lightdm is not using the default already.
Mir's default is already $XDG_RUNTIME_ DIR/mir_ socket. (Also, it's correct that /run/mir_socket is not for apps - it's only for unity8 to connect to u-s-c as a guest (nested) compositor. )
However, $XDG_RUNTIME_DIR cannot be used for the u-s-c, as it'd be against the XDG spec to have rw permissions for non-root on the directory, so phablet user (which unity8 runs as) would not be able to access it. I am guessing that's why lightdm is not using the default already.