The new JACK server comes with a DBUS control interface:

$ jack_control help
$ jack_control ds oss
$ jack_control dp
$ jack_control dps rate 48000
$ jack_control dps wordlength 16
$ jack_control dps capture /dev/dsp0
$ jack_control dps playback /dev/dsp0
$ jack_control eps realtime False
$ jack_control start

To use real-time priority for JACK server and clients, load the mac_priority(4)
module and add the JACK user to the realtime 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_args="--no-realtime -doss -r48000 -p1024 -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.