What you are describing works only with alsa backend. No -X with "jackd -d firewire --help", and it's greyed out in Qjackctl too.
a2jmidid seems to be the only way to go while waiting for Rosegarden to support jack midi.
> Date: Wed, 26 May 2010 01:31:19 -0700 (PDT)
> From: James Warden <email@example.com>
> > I've tried http://home.gna.org/a2jmidid which routes
> alsa midi
> > to jack midi, but haven't had much luck with it.
> I would suggest you try the improved version of a2jmididNo need to use a2jmidid. No need for Rosegarden to do anything.
> which Paul Davis (ardour's main dev) has revisited some time
> ago. I think it is in some git repository so you will have
> to compile it.
> Did you use a2jmidid with the -e option ? it enforces
> hardware ports to be translated as well to jack-midi.
Recent jackd has the option -Xseq that automatically convert Alsa-midi to Jack-midi. In qjackctl, choose "Setup", in the "Settings" tab, at the bottom left corner, for "MIDI Driver" option, select "seq", that will use the "-Xseq" option.