Sounds like a good idea to me.. I quite enjoy working on cars in general so I'm in. (all care - no responsibility tho!) I've got a basic set of tools and a trolley jack that should be able to do most things short of stripping down a whole car.
I would suggest however that people who need the work done definitely bring the beer and food and be realistic about what they want to do on their cars.
Brakes, oil/filter change, coolant change, fan belt change, plug re-gap or change and deck/speaker installation are all easy things that can be done in a short timeframe.
Cam belts, bolting on enhaust parts, installing full stereo systems and the like are not the sort of things we should be looking at doing on the day...
Might I also suggest that this event would be very very weather dependent. Tho depending on the numbers I may be able to organise an indoor event - but numbers would have to be very limited due to space restrictions.
My 2c...