There was a belt fed mode for the original AR 10 rifle. If you watch the promotional Armalite video at the very end they show it firing with belts and then they open the action take out the belt close it and return to magazine fire, If it was possible with the AR10 then it shouldn't be hard to scale it down for the 15 system.
Bellson I'm sorry to hear your friend was screwed by the Cali. government. Sounds like he had a great design and you were lucky to have gotten to test it.