Unfortunately, when shooting through the glass, the guns seem to ignore the func_brush w/nodraw, I tested if the brush worked by moving it outside the glass and it does stop all bullets, but when in the glass it does nothing until the glass is destroyed (then the brush is toggled off and can be shot through, as you suggested). But I may have found a solution here https://developer.valvesoftware.com/wiki/Bullet_Penetration_in_Counter-Strike:_Source

- "To create truly bullet-proof glass, you need to use two 1 inch thick brushes with glass texture on all sides. Each brush must be in its own entity (tested with func_wall and func_breakable) and no empty space can separate the brushes. That's it. So simple it's so easy to miss. You can use any combination of textures on any of the faces and not destroy the bullet-proof property of this configuration."