Yes, I will coat the glass. I think laquer for wood or metal will suffice, I will try to get a thick one.

Not, I sincerely hope, a nitrocellulose lacquer

Commercial flashbulbs use a very thick layer of plastic, probably cellulose acetate or something similar. They tended to have a very characteristic "toasted" smell when fired.

You mentioned making large bulbs; if you do, be very careful: as a kid of about eight or nine, I found some of the bulbs which are the size of a 150-watt incandescent, and fired one by touching it to the terminals of a lantern battery. It blistered essentially all of the palm and fingers of my left hand, even though the envelope didn't break and I dropped it more or less instantly. The radiated power, at close range, is awesome.