My mother-in-law has a flat top glass stove. Everything I've read about them and from what I've seen on her stove up close the only way you repair a cracked is by replacing the whole thing. As I understand them they are made the same way as a flat screen monitor which means by the time you get it in your house it is basically one piece with all layers fused together so it's sealled. You can't take it apart.
I could be wrong.
**** Edited to point that I am in fact wrong, at lest for some flat cook tops. Going to get another cup of coffee now. Carry on. ****