The one piece upper / handguard I saw was the Vltor, VIS upper receiver w/ integral free float quad rail. Vltor Weapon Systems. But I looked and all I could find was 12" , not the 16.1". " VIS Polylithic Upper" the bottom 1/2 is removable for servicing.
I see where you are going with this, just another idea. Don't know if it would still be considered "legal" , but what about those mono framed uppers w/hand guard one piece. If it had a 16.1" hand guard? It would not be attached to the barrel or is that the the way it is spelled out in the ATF...