I thought the same thing when I was trying to flesh this idea out a bit more, but the more I've spoken with farmers, the less I think this is the right way to go. The downside to doing this approach is that for many small farmers contract farming is all but assured. What if it doesn't rain when it needs to? What if you have an odd cold snap that wipes a crop? What if your planned delivery date of particular produce is delayed because things aren't growing as well as they should?
These are all things that happen with a fair amount of regularity on small farms, and things that many good restaurants would be flexible enough to accommodate, but it would sort of negate the benefits of planning out menus at the beginning of a season.
I would ideally love to see this idea extend to foragers as well. It would be kind of amazing to have local foragers post their pounds of morels and fiddlehead ferns and see local chefs snap them up.