Looks like a great project. The lava rocks as per gas grills should be able to take the heat. Assuming you would be dumping them in the bottom of he cookchamber below the water pan perhaps? Thats the only place I can figger out where it would be feasible to place em. If you dumped them under the fire grate it will just rob you of necessary space to get air to the underside of the fire...make it mo difficult to clean out the dead ashes etc. As light and porus as they are not sure how much they would aid in heat rentention. Believe I would make an effort to find some fire bricks if I was gonna pursue this angle. They make some that are half as thick as the standard models. Sure everything in Belgium is made outta bricks. Should be easy to find some.
bigwheel
bigwheel