How about FEC baked potatoes, stuffed with spinach, onions, and cream cheese. Or the same topped with a brocolli/cauliflower/cheese mixture. Have squirt bottles of liquid marg. for them to use as needed. The smokey potatoes would taste great, and I think you could find an easy way to hold them and their topping(s) at serving temp. Also, baked potato wedges with tahinni-lemon sauce. I'll even eat that.
Small calzones would not be much trouble to make, but holding them for servce might be difficult.
I've been looking through my limited supply of vegeterian style cook books again, and there are a lot of Russian and Indian dishes that would be pretty easy to convert to a casserole style item. One hotel pan will do at least 24 servings, so that might get you through an entire day?
I do a mean white sauce based vegetable lasagne that people love. layers of broccoli, cauliflower, spinach, and carrots makes for a colorful serving. They always say they have never seen a white sauced based lasagna, but I would never think of doing a vegetable lasagna with a red sauce. Go figure. Anyway, it holds well and freezes well also, but there might be a market for take home sized portions too since you are at the farmers market and people are in the take home mode. Also works well with smoked chicken added for normal folks.
Lastly, I did a french bread pizza kind of thing for a pool party last week(general idea stolen without remorse from Bobby Flay). Roasted garlic french bread from BJ's, sliced and toasted, then topped with smoked tomato puree, smoked thin sliced red onions, shreded basil, and cheese. I did up 6 loaves(12 pieces) and it held up well even with the tomato puree. The moisture from the tomato offset the drying of the bread. Tasted really good, and it's not so much like pizza that people would expect you to reheat it. It works well at room temp so you could plate and wrap ahead of time. I'm trying to think of things that could be totally prepped ahead of time. It's too hot in Fla this time of year to try and quickly prepare stuff to order for window service. That's almost like work.
BTW, what does Jack think of all this vegeterian stuff? Is he still speaking to you?