The OP is looking for weight. Your solution does not have that. He is not looking to solve a problematic leakage.Foaming the tires would be a solution.
All valid points I believe.A foamed tire may have weight but it will not pull as much as an iron ballasted or liquid ballasted tractor.
The foam filled tires do not have the long foot print that a properly pressured tire does.
And the ride is terrible if not on a smooth concrete surface.