i would strongly disagree with this! the points where the seat brace bars go have like 4 spot welds holding them in place. In a crash im pretty sure they will just rip out.

i have spoken to an engineer and he said that your seat belt bolts need to be able to support over 2 ton of lateral force, put 2 ton on a rear seat brace and see what happens??

your safest bet (and i think what cams requires) is to drill a hole and have a large plate under the car (i would make it 150mm x 150mm) and bolt your harness to that. also at the correct angle