Hello William,
you identified the problem correctly. Unfortunately cgx does not create internal element faces and can not deal with them. I see only the possibility to create a set of the relevant elements. Then write this elements with the send command to the filesystem (send set frd c)and open this file again in a second cgx session. There you create a set of the now visible faces (qadd set t1) and export them in the usual way.
Best regards,
Klaus