This may come a bit late for 3.0, but I’m suggesting renoise should use jack server’s dbus interface to start/stop the server, and manage it’s settings.
This way systems with pulseaudio would work alongside jack without any extra tweaking (in case module-jackdbus-detect is provided for pulseaudio). Also audio server settings (sample rate, buffers, etc) could be made easily manageable directly from renoise.
However some distributions/users might still use legacy jackd, so there should still be a fallback for old behavior in case jackdbus isn’t available.
currently you can achieve this fairly easy with a startup script for renoise. Implementing this in renoise would make it easier for non technologically oriented users to set up jack, especially alongside pulseaudio, which is often required by desktop environments on most modern linux distributions.