Inspection shows that its composed out of 3 parts:
They are encased into the Func_wall entity thats the window bar texture.
The two other parts are solids.
The front solid has a SKY texture on it, while the other side is BLACK. The second solid has BLACK on both sides.
This way VIS is blocked, but i tried this before and all i got was a black box behind my sky...
Just tested it, it works:
BSP FileRMF File[Halflife.wad | cs_dust.wad]