I haven't set up my kettle with firebricks; I use the Smokenator, which, while unpopular on this forum, works in essentially the same fashion, restricting the charcoal to just one third of the grate and using a water tray as a heat sink. The Smokenator was in fact designed to allow a relatively large turkey to be smoked on the remaining portion of the charcoal grate; a firebrick setup might reduce that available space somewhat, but I don't see why a smaller bird or even a rather large butt couldn't be cooked there.
I once smoke-roasted five whole chickens in my kettle, two directly on the charcoal grate (I bought a replacement charcoal grate just for this purpose) and three on the cooking grate. If you're worried about it, a roasting rack in a roasting pan or disposable foil pan should be enough to keep the food mostly ash-free.
Of course, some allowance may need to be made for a difference in cooking temperatures between the charcoal and cooking grates. A great use for this space is baked beans, which can then catch the drippings from whatever you're smoking above.