If the top surface is not going to be used as rough as a kitchen countertop, how about using a reed spline, rather than grout, to insert between the tiles. Reed spline is inexpensive, fairly strong and flexible. Its easy to cut/miter and glue into the spaces. Reed spline holds pressed in cane in place, without any problems, so I would think it would be reasonable for your use.

If you use reed spline and it eventually doesn't work, then removing it, to use something else, would be easy. Your investment in this "testing" of spline would be nominal... spline is really inexpensive.

A spline could be stained, for contrast, also, for that consideration option.