I used to work with glass, and my parents still do leadlighting as a hobby.
The glass shouldn't break, as guitar guy said. The reason for this is that it is supported from the bottom. If the nut wasn't installed it would be easily broken, one drop on any hard surface would crack the glass where the slots are cut.
I'm very sure this would work on a strat guitar, where there is more support than a gibson style nut, although if a strat style nut made from glass works, i'm sure a gibson nut would too.
Of course, the nut slots would need to be perfect, as their is still a risk of the nut breaking outwards on the two E's slots.
I could cut one up and slot it, but i don't have any guitars i would like to pull the nut out of right now to try it
Rgman.