I've been on only a couple of teams in with Paramedics in my 40 years of racing Baja. Those teams were large factory teams from the days gone by and no longer race. Commonly there are Paramedics who area also racers or pit/chase crew members of teams, but their medical support is secondary. Here in Southern California there is a group who provides support to local desert races called Rescue 3: http://www.rescuethree.com/
You could also contact SCORE International directly to see about being part of their medical support team. http://score-international.com/
There is also Best in the Desert who puts on races in California, Nevada, & Arizona: http://www.bitd.com/
I believe that Rescue 3 may do the support for at least some of the BITD races.