I'm still in med school, but when I worked in EMS, all of our medical directors worked in the ED full time. Most were EM boarded, though one of the services had an IM boarded doc who worked in the ED, and when I worked in a rural service the medical director was a FP doc who worked in the ED and in his own outpatient practice.
Medical direction is not a full time job except in the largest of urban ambulance services (In Philadelphia I'm pretty sure the Fire Department medical director is an attending at HUP, for example), so I can't imagine a better setup for being an EMS medical director than being a boarded EM doctor working in an ED.