Can you substitute yellow summer squash for the zucchini? I have a boat load from my CSA that needs to be used!

Totally interchangeable.

I picked up a bunch of yellow squash (it looked better than the green) and oriental eggplants from my neighborhood farm stand. I cooked them on the grill because it's so hot here, but otherwise followed the recipe. Really delicious.

I imagine this method would work with summer squash, eggplant, lettuces (heartier varieties like endive and romaine), cauliflower steaks, asparagus, fennel, even brussels sprouts. Of course summer squash and eggplant would be the closest in flavor, but this is a method to play around with!

Absolutely! Any kind of summer squash will work, but smaller ones will taste the best.

In my experience, summer squash varieties (including zucchini) are pretty completely interchangeable with each other, with fairly subtle variations in flavor in texture. For this recipe, I think you'd want squashes that aren't too watery or overgrown (no war clubs!), but any variety would be delicious.