I was given a small pile of solid red bricks a couple of days ago (thanks again freecycle!) that have the corner chopped off in a 45 degree angle. They probably have a proper name, if you know your bricks. They look like they'd be useful for building the inner arch of the doorway, so long as they can take the heat from the fire. I was burning a load of garden waste in the incinerator anyway, so I stuck a few of the bricks in a tower in the middle of the fire to test them.
The fire burned for about 3 hours (a lot of stuff to get rid of), then burned down overnight. The embers were still red and glowing after 6 hours, so those bricks got pretty hot. 24h later they were still too hot too touch, and were almost too hot to hold with my thick welding gloves on. The bricks survived fine - didn't seem any worse for wear after their cooking. That's a cooked one on the left - the one on the right is uncooked. I think they'll be fine for my doorway arch - the chopped off bit should allow the inner door to fit nice and tightly. And they're free!