The only reference I found says that the seats on the 747-100 and 747-SP weigh 200 pounds each.

The Basic Empty Weight given by the manufacturer includes seats because they are considered to be part of he fixed equipment.

It might not even be possible to fly without seats if there is a minimum weight that you can't go below. Basically there are handling problems when the airplane is too light; it responds differently to things such as turbulence, asymmetric thrust, and control surface movements. I found an NTSB report about a B707 that crashed partly because it had the seats removed and was too light.