Geoff, I have prepared meals with all / most dinner menu items sourced from the farm. The low carbon bit was of topic but it came off more like a family day at the farm. I had invited families I knew who had invested in renewables, solar homes and vehicles. Making food part of the energy discussion is difficult.
For a moving event different menus could stress the energy point by intentionally putting together one menu with ingredients flown in and shipped from various places around the world. Here in Calif. that might include (French) endive , hothouse sweet peppers from Belgium, Italian truffles, Chilean fruit compote, and a bottle of Shiraz from Australia.
Another might source as many seasonally available items as possible from one or two local farms. A third might try to source products that can be shipped by rail transport (,bulk commodities like grains and potatoes ) . The third menu might even beat out the local one energy wise . Both menu two and three should stand in sharp contrast to the air transport of food .
Scale also can decrease food energy costs and it is an important part of the discussion sometimes overlooked in the eat local food message.