Been thinking about this for a while. Decided to try it and the result turned out pretty darn good. Everyone who tried it gave it a good review. I took pictures but somehow managed to delete them so you will have to take my word it looked good too.
I had dough for a crust from my last pizza effort. 4 cups flour, 1 /14 cups warm water, yeast, let rise and flatten out. Makes two 12" pizzas
Sauce: I made a garlic parmesan white sauce. 2 Tbs flour, 2 Tbs butter, 1 1/2 cups whole milk, 1/4 Tsp salt, 1/8 Tsp pepper, 2 Tbs garlic powder, 1/4 cup parmesan. That turns out to be enough for 2 pizzas. melt butter, stir in flour, add milk just like making any white sauce, add spices and then parmesan stirring constantly. Spread sauce on pizza dough.
Toppings: I put on about 4 ounces sliced olives, 4 ounces mushrooms and chopped, not ground clams (enough to cover the pizza, I put it on pretty thick, but I removed the necks as I thought they would be tough)
Cover it over with mozzarella and into the oven at about 425 for a half hour. I had to turn it down to 350 and cook for another 15 minutes because of moisture in the clams.
What I would do different is I would put the chopped clams in some paper towels for a while and try and soak up some of the liquid. It wasn't soggy but I had to turn it down and give it time to cook off the water while not burning the bottom of the crust.
Even though I used a lot of clams it wasn't over poweringly clammy so I would not worry about using too many clams.
I never heard of anyone doing this with clams, but I will be doing it again.