H&S would be covered in the planning stages by Risk Assessments etc, very often it's a give and take process, as long as they don't think they're dealing with some fly by night cowboys then they will be receptive to proposals.. H&S is all about mitigating risk, if it takes double thickness netting then that's what it takes.
It would be the biggest challenge admittedly but not insurmountable.
For example there have been golf driving ranges slap bang in the middle of London, lots of netting, problem solved.
If you can prove it's safe to the H&S officers, which wouldn't be difficult, then the next challenge is raising enough cash to cover the safety aspect. I think you're right though, standard event netting wouldn't cover it.
Presentation will be the key here. When it's taken to the Mayors office if it looks amateur it won't happen. If it looks pro it will be on the first step, then you get into the details. LMS is a professional and knows how to talk the talk (as far as i know).
I've just remembered I have a mate in the Mayors office who was involved in organising the events surrounding the olympics. LMS, drop me a pm and we can see if it's worth talking to him yet.