Just an observation from a home cook: mesclun will wilt. Traditional Greek country-style salads (salata horiatiki), which most of the rest of the ingredients suggest, can stand up to the heat because there are no leafy greens in it.
The ones I ate in Greece when I visited in summer had tomatoes, cucumbers, onion, sometimes olives, red or green bell peppers and feta cheese chunks. (I don't remember eating a leafy green salad during the month I was there except on the cruise ship.) The usual salads were dressed with olive oil and red vinegar, a little salt and pepper and fresh or dried oregano. That was it. Sometimes it was served as a composed salad, other times mixed in the kitchen. In the heat this sometimes breaks down but it doesn't lose its delicious flavors. Marinated or fresh button mushrooms would be a nice addition because they will just become marinated and not spoil.
That said, I'm sure you can take of liberties with the veggies you use so long as they won't wilt as they slowly marinate. Zucchini, prepared eggplant (lightly marinated, I guess), capers would also be very nice. Cruciferous veggies like broccoli, kohlrabi and cauliflower might work just fine but that would be a departure from a traditional Greek salad.
I'd be interested to know how it works out.
Greek salad ideas? 
