Looking at the mailing list it's seems not possibile configure freeradius to use a CA chains certificate, instead of a single CA root.
Since also Comodo distribute server certificate in that way I wonder if this could be managed in future release of freeradius.
Comodo has been choosen by Terena TCS service that is used by almost all educational organization in Europe .. and many of them, like mine, use also FreeRadius.