Many local enterprises already look for opportunities to support each other. The B2B Buy Local Pledge formalizes that commitment by letting businesses earn "Pledge Points" for buying local, sustainable goods for their daily operations.
Perhaps the problem is that 10% or 20% of customers really do want to eat that much food, so in order not to displease anyone, the restaurant serves the large dish to everyone. This could be solved by offering small/large* versions of every entree, with the large version costing somewhat more.
*clearly at hip restaurants this could not simply be called small/large but would have to be termed "piatto piccolo/abbondanza" or represented with stylized hummingbird/pelican graphics.
Sara, I'm glad you see potential for "interaction and camaraderie" since that's what I would love business owners to get out of this - the chance to interact with their peers and customers and talk about issues like this. I agree there should not be some hard and fast litmus test (if a national chain features locally made products that should be supported, though I can't think of too many examples of this.) Rather it's just making a commitment to consider the issue. The opposition is not to global trade per se, but to "lowest common denominator" globalization.
Controversial/gorgeous/trashy, this project has been around for 25 years and made the transition from the city knocking it down to (kinda, sorta) establishment. It is now several square blocks of interactive outdoor art, plus art education programs. They might make a very cool partner for working with companies and artists who want to contribute.
I love the idea of multiple groups each working in a different neighborhood with different styles, either in competition or just in harmony. Those colors above are gorgeous!