Today, it is very simple to launch the Jack Audio Connection Kit daemon with realtime scheduling, with a no-root user.
When Jack Audio Connection Kit is installed, it adds specific "limit" to jackuser group, into Pluggable Authentication Modules (pam).
Cf. the /etc/security/limits.conf file.
So the only need is to add the user which must need to launch the Jack Audio Connection Kit daemon with realtime scheduling into the jackuser group:
/usr/sbin/usermod -G jackuser "username"
Then, use the -R option when launching Jack Audio Connection Kit daemon:
jackd -R ...
N.B.: those instructions have been performed (at least) under Fedora 8