Taco Pizza with Guac
Don't cook it - at least I wouldn't. I would drop guacamole on top after its out of the oven, so not cooked, not on the side, just simply on top. Refried beans are a common "sauce" base for Mexican pizza so you might think about that as well.
I like to high-heat grill avocados on occasion but I think the heat of the oven would turn the guacamole mushy-gray.
I have a recipe for Mexican Lasagna that calls for sliced avocado to be layered with the other ingredients before you bake it. The avocado turns out really well when baked, but it does soften and become creamy and rich. I think a guacamole base on a Taco pizza might come off as very rich. The beans are a good suggestion for the base.
I don't think you should cook the pizza with guacamole. While sliced avocados might work in a mexican lasagne dish, it would be because they are not exposed to direct heat and air while cooking, thus retain their color. I would expect the guac on a pizza to blacken and become unappetizing-looking. I would serve the guac on the side.