The port net/p2p/transmission-daemon is an example of service that utilizes <name>_user rc.conf setting. Also, it supports writing debugging logs to pre-opened (redirected to a file) STDERR by means of environment variable TR_DEBUG_FD=2 but our framework uses "su -m" for such services executing root shell that defaults to stock csh (contrib/tcsh) that closes its file descriptor 2 at start.
Note there is some kind of work-around if we do not use login classes and/or limits for the service, then add to /etc/rc.conf
So "chroot -u transmission /" is used to substitute user instead of "su -m" and chroot(8) is documented not to use root shell but SHELL=/bin/sh. And /bin/sh keeps redirected STDERR open in such case.