You can do this with a separate FreeRADIUS server to use as a proxy. in the realms configuration you would simply put a regex for the MAC addresses to instruct it to forward to your MAC address radius servers, and then specific domains (or even DEFAULT) for the 802.1X sessions. This is also a way to segregate different domains to different RADIUS servers.
If you already had freeradius servers you can chain them, and pull out, say, the 802.1X sessions to proxy to an upstream server and then auth the MAC auth sessions on the existing server. (this is exactly how NAC does it when you choose auth MAC locally).