Cut a thin slice from the stem end of each tomato and with a melon-ball cutter or small spoon scoop out the seeds and inner pulp, leaving a 1/4-inch-thick shell. Sprinkle the tomatoes with 1/4 teaspoon of the salt, invert them onto paper towels, and let them stand for 20 minutes.
In a food processor fitted with the grating blade or with a hand-held grater, grate the zucchini and the summer squash coarse. In a colander toss the squash with the remaining 3/4 teaspoon salt and let it stand for 20 minutes. Rinse the squash under cold water, squeeze it dry in a kitchen towel, and in a bowl combine it well with the olives, the Feta, and the egg yolk. Stuff the tomatoes with the squash mixture, mounding it, put them in an oiled small baking dish, and brush the outsides of the tomatoes with the oil. Bake the tomatoes in a preheated 450 degree F oven for 10 minutes. (The tomatoes may be baked with the hens during the last 10 minutes of roasting.)