Matt -
First, let me dispense with the hero worship -- I think you have done more to advance the PO meme than anyone else I have run across. I think your message rings in its accessible tone. You are not a shrill scream but a reasonable conversation kin to that my friends and I have over drinks, and for that reason I believe your words stick a little bit more with mainstream America than some of your colleagues. For that reason, yours is the site I send newbies to first.
If you find yourself in the Chicago 'burbs, ping me and let's get some beer!
Had the land belonged to the city, ergo the taxpaying community, I'd line up in your corner. The town where I live has community garden plots which my mom and dad used to use until we bought land out of state and built a monster garden there. I think community farming and sustainable agriculture is a hell of a great idea as well as something that is so very, very, necessary.
Nuts to the city for selling it when they had a good thing going. If the group is organized, they could save the money they'd pay the lawyers (

) and find some additional property nearby and make it their own. It's not the best solution but sometimes you have to make do.
If we cut the "peronsal property / common good" salami too thin it turns into a slippery slope, IMO, where everyone loses. Look at the case where the government evicted homeowners where they want to put retail space.
Have you been watching what's been going on with the greening of Detroit? Sustainable farming within city limits. Could the LA folks learn something from the Detroit folks?
I think the safest way to proceed it to raise public awareness and develop sustainable urban farming with the blessings of the cities. That means debate and democracy...which will take forever.
But do you really trust the government to provide solutions for us?