This post indicates that a travel consolidator did have access to non-revenue inventory but this access was terminated, date as of yet unknown, and the poster suggested they subscribe to EF to gather such information, which they did. This indicates to me that there was non-revenue inventory available until some recent date on the GDS/CRS. It also indicates that EF apparently was gaining their data by means other than the GDS/CRS the travel consolidator was accessing.

As I once was able to peer over my travel agent's shoulder as she booked award tickets from some non-sensical (to me) gibberish on a green CRT screen, I considered the option worth considering. She did mention things like 'upgrade inventory' but that was all alien to me back then.

If it turns out there are no options to address this issue, that's an answer. IMO, the issue is too young to currently jump to that conclusion.