The new JACK server comes with a DBUS control interface:

$ jack_control help
$ jack_control ds oss
$ jack_control dp
$ jack_control eps realtime False
$ jack_control start

Only root can grant real-time priority to processes for now, using rtprio(1).
Upcoming FreeBSD releases will have a MAC policy with a realtime user group.

Memory locking has to be allowed in /etc/login.conf or ~/.login_conf. Set the
resource limit ":memorylocked=unlimited:" and don't forget to run

# cap_mkdb /etc/login.conf

It's still possible to start JACK server as an RC service for a dedicated user.
Note that only one JACK server can be run at a time. An /etc/rc.conf example:

jackd_enable="YES"
jackd_user="joe"
jackd_rtprio="YES"
jackd_args="-R -doss -r48000 -p1024 -n1 -w16 \
            --capture /dev/dsp0 --playback /dev/dsp0"

Official JACK example clients and tools are available as jack-example-tools.

JACK 1.9.22 introduces a latency correction fix.
Latency correction parameters have to be measured again after an update.