I figured out rxfax wasn't doing it, but rather txfax on the sending side.  The documentation says to set fax_header to '_undef_', which I could get to work as expected if I did it in spandsp.conf.xml.  I couldn't get it to work if I did it inside the originate {} command I send from the command line.  If I set fax_header to anything there, even _undef_, that's exactly what it sends.  I have to skip it on the command line to get it to suppress itself.  Works for me.

- Jeff