If the answer is "yes", can I do this for ANY type of MIDI synth (i.e. qsynth, VST, etc) or not?
you can export audio as it's rendered by any type of MIDI instrument plugin; else for external instruments, which aren't inserted as plugins (eg. qsynth) you'll have to bounce record their audio output in real-time as usual for most JACK client applications.
Does qtractor support this type of "freezing"?
yes. it is done as exporting audio tracks.
If the answer is "yes", can I do this for ANY type of MIDI synth (i.e. qsynth, VST, etc) or not?
you can export audio as it's rendered by any type of MIDI instrument plugin; else for external instruments, which aren't inserted as plugins (eg. qsynth) you'll have to bounce record their audio output in real-time as usual for most JACK client applications.
byee