I can think of any number of bikes that have similar or worse mounting systems.
I don't see a problem with welding one up if you're a competent welder. That said, losing a footpeg while in motion would be a real danger - especially since the passenger would likely try to either get a perch on the exhaust, the chain, or the wheel.
There have to be hundreds of them available online - I know Bent Bike (the local wrecking yard) has probably close to 200 sitting in a pile up on a shelf.