I suppose legally it would vary by state -I would probably call the Board of Medicine, ask to speak to an attorney serving on the board, and talk to your free clinic director.
The free clinics I have worked in all had umbrella policies protecting their volunteers, and physicians receive tax breaks for donating their time and expertise. Several of the clinic physicians were specialists seeing pts for general concerns, and several were retired physicians, so they were not keeping up a separate malpractice policy.
The clinic patients were vetted (had to meet income requirements, etc) and any physician at any time could ask to discontinue to see a particular patient if the patient was abusive, etc. My experience in general was that if you were a specialist, pts would be assigned to you who had problems in that area (so our orthopedist volunteer would end up with lots of back pain pts).
We also had several specialists who saw patients in their offices, and a local radiology practice that read images for us. The hospital and local labs took care of a lot of specialist services/ancillary costs (OR, path, ED, etc) -this makes a lot of sense, because the idea is to cut back on ED visits and really expensive problems by getting these patients into regular preventative services, thus preventing DKA, MIs, etc and saving the hospital $$ and ED overcrowding with sinus infections, etc.
You certainly could volunteer for a free clinic w/o a license, board cert, but your services would likely be limited accordingly.
One of the things that helps w/ the whole malpractice thing is that free clinics in general do not allow prescriptions for controlled substances.