The Renard seems like a neat controller. I looks over the communication specs briefly.
I think just plopping in a new driver will be unsatisfactory for RG.
But yes, RG is geared to accept a new driver if you are willing to write one.
In RG svn repository you will probably need the driver to interface with:
trunk/rosegarden/src/sound/SoundDriver.h and .cpp
for MIDI we use:
trunk/rosegarden/src/sound/AlsaDriver.h and .cpp
But given the brief read on the communications protocol, I think RG would need new events to handle your calls.
In RG everything is an event. Quarter Note, Quarter Rest, Controller Event, etc.
You can add new events as well.
In RG we put events into segments. A segment in placed on the main canvas (segments are placed in the composition). Then the segment is opened in an editor so that events may be placed in them.
So you will need to decide how to allow users to add these events in each of the views: Event editor, Matrix / percussion view, Notation view.
I think this could be a large project to get these events routed to your controller using RG.
I don't think it is impossible, but using our current events and sending them through the AlsaDriver will not work. Some of your control codes will not be honored, and we have a very unfriendly system exclusive (MIDI system exclusive) event that will not accept the codes you would like to send.
Even with all of this work, the results may be a bit unsatisfactory in RG.
I hope this helps.
You might be better looking into seeing if you can port Vixen...but I don't know what that is all about.
If you already have the communications worked out you might consider a custom app or something specfic to your needs if you can not find a OS alternative.
PS -- Maybe some else has more insight into this than I do.