How much heat can a buttered baking croc handle?

Since it has a lower melting point than cooking oils, probably because it's not pure...has milk sugars, etc. in it.  Same thing with extra virgin olive oil...but straight or light oilve oil works.  Clarified butter/ghee might do it for you.

In cooking--pan sauteeing, for example--I have used a combo of butter and oil, and this increases the smoke point temperature.  So you could try a blend of both butter and oil.  Or maybe a specialty oil--peanut oil can stand high temps and yet has a different taste than canola.

If you are using a well-seasoned cast iron utensil, the seasoning process forms a non-stick type coating anyway...and if your cast-iron is smoking-hot, you shouldn't need to oil it.  The dough should immediately crust when it hits the hot metal surface, even faster than dough hitting a baking stone, since the metal will hold the heat better than tile.  So really, you should be able to get away with no oil at all.

Windi 

I use a Kitchen Aid dutch oven, enamel over cast iron, inside and out.  At first I oiled it, but after the first few times I tried without once, and have never oiled again. The only difference I saw was that I had to scrub the oil off the inside of the dutch oven.  The bread never stuck at all.