The opening of 12" seems a bit small to me to load and unload pizzas in a comfortable manner.
If you have a router, Dremel and the like, I'd cut open the front perhaps up to 16 inches while keeping the size of your peel in mind. But just like Don mentioned, if it's glazed you're up for some major chipping and, perhaps later down the line, cracks.
I don't understand this statement, though...
Chiminea's weren't designed to cook pizza, the Titanic wasn't designed to sink, and deep sea drilling rigs weren't designed to leak. The world is an imperfect place and I love all of it.
Not the world's unpredictable or imperfect, it's us, the humans, who make it unpredictable or imperfect. The more we f**k with Nature the more it comes back and f**ks with us.
Plain and simple.