You certainly can use fire bricks, but the main advantage of the clay saucer is that the shape conforms better to the shape of the cooker, which means that you limit the hot spot on the outside edge of the cooking grate. A fire brick is a square peg in a round hole, so to speak.
That said, I have used fire bricks when converting to a cold smoker. I throw a few of them in the freezer, put them on top of the frozen clay saucer and then put a metal bowl with ice water on top. with just a couple of lit briquettes, you can run cold for a fairly long time in cool weather. All that cold mass takes forever to heat up.
Just don't let your family member catch you putting bricks in the freezer or they may reconfirm their suspicion that you have gone off the deep end.