Hi Sam,
did you find documentation about FAX detection on the NM-HDA? If so please forward because I'm not aware of such a feature.
One possibility with CME would be to use the B-ACD script. Callers would hear a greeting, then a menu that you can use to send to fax port, or possibly if they don't enter any choice they are sent to the fax by timeout.
Hope this helps, if so please rate post!