Use switchable textures for the windows? It's probably not what you want, but just a thought
Or maybe, try this by having:
-Door1--the door frame
-Door2--the glass. make it passable--can you make doors passable in HL by adding the field through smartedit? I know you can in CS, soe maybe.
However you trigger the glass to be broken--tons of ways you could-, env_render the passable glass door invisible, and spawn some breakable gibs with an invisible breakable or gib shooter (I can't remember if an invisible breakable will yield
visible gibs or not)
If you can't get it to work, upload the door to problem maps that you want to have the breakable glass, tell me how you want it to trigger broken, and I'll try to do an example map. Also, PM me soe I know it's there