I've used regular bricks without issues as well as river rocks....I think the issue with brick exploding happens when people take a brick that's been sitting in the dirt, soaking up moisture for years and then toss it in a hot grill....pretty sure if the bricks are stored dry there's no issues....heck my fireplace is made out of bricks.
I would bet that the inside of your fireplace is lined with firebrick.
Either way, I know people use regular bricks and other stuff...All I know is what I use in the PBC. I can't confirm that a rock or a regular brick will work the same as far as radiating the heat. Fire bricks are very dense and hold heat for a long time. ....What your doing in the PBC by blocking half the coal basket to use less coal yet maintain the same temp is different than banking coals in a weber to create indirect cooking. Other stuff may work but again all I've ever used is firebrick.
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