Lots of people use brew belts on their glass carboys. Please note that my house is such that I do not need to, so have no personal experience.
One word of warning...carboys tend to break when exposed to quick temperature changes, eg a cold carboy is washed/rinsed in hot water. I suggest not putting a brew belt on a cold carboy. Warm it up somehow first.
I thought I had read that people use them on glass, but I wanted to make sure, my cellar is 60 deg all year long, but it would take too much to heat it up to 75-80 just for the primary and secondary, I thought with the belts it would fix that problem. I'll make sure they are room temp before I put them on.
I have been using the brew belts on my carboys for many years with no issues at all but like Cp said dont put one on a carboy that is like 40* or the temp change could crack it! The lower you go on the carboy the warmer your must will get or at least thats what people state. I just throw mine in the middle and get the wine going good and usually once its going good in my basement the fermentation will make enough heat to keep it warm till it starts to slow down and then I turn the brew belt back on.