Not feasible, unfortunately. I strongly recommend the workflow you described for people who want to use JACK. The only reason to use JACK is to allow inter-operation with other JACK clients, which means that starting JACK from QJackctl or Cadence makes the most sense. That way, Ardour doesn’t try to start JACK itself and everyone is happy.
Ok Paul, thanks for you answer. I guess the other reason to use Jack is no longer relevant, namely automatic latency compensation. It seems that Ardour now lets you define a value for this in its audio engine dialog.
You can do that with the JACK backend also.