Tile over brick...with a problem
You want something with close to the same thermal and moisture properties as the rest of the surround. Best wound be a piece of brick. You could bond it in with thin set since it is not really structural.

If none are handy, you could use pieces of tile layered using thin set to glue it in. Just make sure none of the pieces stick out more than the brick. 

You might want to put a skim coat of the set over the whole brick face before tiling to give you a smoother base for a more even finished surface.  This ski coat would cover any irregularities with a pieced tile patch.