I have a Colt upper in 7.62x39 with a flash suppressor and no bayonet lug. Can this be put on a postban lower?

You mean a post-ban lower, perhaps?  No - not unless you remove the evil pistol grip, or make your magazine non-removable.

Yeah that would work.  Or you could weld the chamber shut and weld the barrel to the reciver.

Or, You can permanently attach a recoil compensator...as opposed to a flash suppressor...and remove the bayonet lug. You are only allowed one evil feature besides the detachable mag on the AR, which is the pistol grip. To permanently attach the compensator, you have to weld it on, 1200 degree silver solder it on, blind pin and weld, etc. You can also remove the muzzle device and threads completely, provided you don't go below 16" on the barrel.

The upper and lower must me preban.  Or it is a $200,000.00 fine plus 10 year in jail.