I use sic powder to flatten glass and Pyrex dishes so it does cut those materials quite well. Question is how flat do you want your lapping surface? I would think it would take hours of lapping to wear a few thousandths off of your glass lapping plate.
By the way you want to lap in a figure 8 using as much of the plate as possible.
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 .
I think you'd need a really accurate sharpening system to notice the dishing of the glass in leveling of the stones. I would think that would be the least of your worries in most cases. I use the Edge Pro leveling kit and have no problems (that I know of).
After heavily flattening my large SiC stones about 10 times on glass with 80 grit SiC powder, I could notice some dishing on the glass plate and the stones started to come out slightly dome shaped rather than flat.
glass plate and granite tiles are cheap. Replace the base when worn too bad. I see no reason both sides cant be used first, then replace. Maybe go to a glass shop and see what scraps/leftovers they have that you might can get for much cheaper. You can buy single granite tiles at lowes and home depot too. They work good, just verify the tile you pick is flat before you buy it. Most are plenty flat for our needs.