Not sure how long it would stay flay. Quick search showed glass as an average hardness of 5.5 on Mohs scale. Silicon Carbide 9-9.5 on Mohs scale, so eventually the powder is going to abrade away the glass.
Maybe another option to consider-I use a cheap $12 granite tile from HD to lap my stones my SiC powder. This works better for me, because I would break the glass (clumsy). Yeah the tile can break too, but I just set in my sink, load with powder and go- as long as my pencil lines come off the stone, I should be flat enough .