I don't know anything about permission, but I do know that when we offered a 'veg patch share' we ended up being taken advantage of, tools broken, and way beyond the fair share of produce taken. These were 'friends' so we had not thought to have any formal agreement. However, I would suggest that they provide all their own tools, seeds, etc, although you would need to ensure a water supply. You might also want to specify acceptable times for them to be there, or rather what times were unacceptable, parking, access etc. I know it sounds petty, but your altruistic desire to help others have access to grow their own crops can end by blowing up in your face.
With those caveats, I think it's a lovely idea - good luck