The rack has a solid mount point at the front where it bolts to the subframe through a hole in the fender. However, the rear mount screw is not into the subframe but into a captive nut under the fender itself. If you were to sit on it your weight would soon see that fitting fall through and rub against the tyre.
When you put a pillion seat on the bike it bolts to the front mounts but sits on the fender with rubber bumpers to spread the weight and the rear bolt is only used for positioning.
However, you could look at fabricating a seat that has legs that bolt to the rear saddlebag fittings, which do bolt into the subframe. I think those bolts are only 8mm, perhaps 10mm, (it's a few years since I had a Scout) and you'd need to check sheer resistance to ensure they have the capacity to take the weight of a pillion on a rough road. The bumpers might still be necessary.