I have seen ly.com offer cheap consolidator fares on domestic US trips. Last summer I saw an AS sector BOS-SFO marketed through their channel which was about $100 cheaper than any public fares. No idea how this worked out commercially, but it was legit - eticketed on 027 stock in a normal revenue fare class, just using an agency fare.

It's a good question to wonder whether other private agency fares might circumvent accompanied-travel restrictions.