Its too late now but here's my 5p worth.
With natural clay brick tiles we cut up wall tiles into strips and use them as spacers - having a glazed surface they come out of the joints easier.
If the effect is to be a brick bond recess we tend to point the joints rather than grout them and even have a recessed joint - at 15mm thick you can just rake the adhesive in the gap smooth.:mad2: - just thought the brick wall was apt.