I think pits are still used, our loacal has one (if it's the same typre i'm thinking of) The planning permission to modify the building would be a pain in the arse though.
I think there are 2 main markets this idea appeals to, you've probably already figure them out:
Enthusiasts - These people like to work on their own pride and joy, they don't trust mechanics. They enjoy tinkering at the weekend etc. Why would they want to come to my place instead of their own place? Well ease of use, possibly dont have working space (unlikely), socialise with other enthusiasts, and the obvious one, don't have ramp access.
Teens, younger people - Yup these would make up a huge market segment imo. They lack tools, accesories, ramps (duh), even haynes manuals

.. These are the customers that will make the business profitable throughout the day as the bunk off uni or college (like me), they spend all disposable income on their car, they like to fit their big exhausts and bodykits..
What it all boils down to is insurance, you can exempt liability apart from the obvious death or personal injury but it's the risk, my dad mentioned something but i forgot and it started to sound like it would be a possibility, until i'm clued up on it all i'm not sure.
No the staff would directly intervene with anything, they are supervisors, they do NOTHING else. It's the simplicity of it all.