Once again, Heather, I would point to Fifth Season Co-op in Wisconsin. There are several ways of learning more about it. 1. Talk to the co-op. http://www.fifthseason.coop (2) Ellie Bomstein did a strong paper about their early days when she was a grad student. (3) There is a brief mention in my book, Building Community Food Webs. (4) I am happy to tell you what I know.
Key elements were that the growers formed a cooperative and then invited reps from the buyers (hospitals in this case) to join the board of directors so they would have similar skin in the game. Each fall, the growers would meet together and basically bid on producing a certain amount (acres? pounds?) for the demand that had been identified by the hospital and schools (who could not join the board because as public entities it would be a conflict of interest to be asking for bids from themselves). My favorite part of the negotiation was when the farmers sat down directly with the food service directors at the schools and came up with an original product that the farmers could raise, a local processor could process, and the schools could cook in chafing dishes without much labor. This created a brand new spiced root crop medley that has been used at conferences I have attended as a tasty meal. It is my understanding that over time the growers moved away from planning together, and I have my guesses why that happened, but it is not something I have direct knowledge about. The co-op also purchased group liability insurance and saved a ton of money on that.
Otherwise, you might also want to check with WOLF Cooperative in Indiana. That is an Amish grain milling cooperative (see http://www.crcworks.org/innetworks16.pdf for an overview). I do not know if they plan production together, but it would not surprise me if they do. The co-op board are primarily Amish farmers and they have huge expanding markets in organic dairies and organic chicken bsrns nearby.
One could also check with the various Amish and Mennonite farming communities that have built their own auction barns for selling produce from their own farms. Some of these groups may do some crop planning together.