Hi Julien,
The easiest way right now is to use my old alsaseq bridge:
http://konstruktiv.org/files/software/jackmidi_alsaseq-0.3.tar.bz2
There's a patch for jack to add rawmidi driver to alsa backend:
http://boudicca.tux.org/hypermail/jackit-devel/2007-Jan/0065.html
I'm slowly finishing alsaseq version of a driver and will announce
when it will be ready (RealSoon(tm) I hope).
Feel free to write if you have any problems.
Regards,
Dmitry.