Great idea and good observation for all that unused land!
When you talk of this it brings to mind the old monasteries and how they use to have their big gardens for food and their herb gardens for medicines.
And with the churches also sometimes serving as a place of refuse, what better way to serve the community then by meeting their physical as well as their spiritual needs.
I've been struggling to try to get a school garden set up in town here and we get it all set until the very last permission and the bureaucracy just doesn't seem to be able to give that final okay - frustrating!
My way of thinking was if we can designate all this space for parking our cars (which is our means of transportation) why can't we designate more space to grow our food (which is what sustains our physical body).
I also look back to the war years and the victory gardens when it was fully supported for any arable land to be put into productive food growing - bring that back again with the permaculture methods for a more sustainable longer term, bringing us back to more like the garden of Eden!
I'm looking forward to see how this plays out! Good luck with your endeavors!