Nope... everything which is tied to an entity wont block VIS
warning ascii edit

something like this would block vis into a big room
?????|???|
___|???|___
|??
______???|
|??
|_____|???|
|?????????????????|
|??????BIG??????|
|????ROOM????|
|_________|
The red would/should block VIS into the room (its a wall from floor to ceiling), also 90 or 180 degrees turns also would block VIS.
you can see if the visblocking works by typing the following in the console:
gl_wireframe 2
this only works in a NONE multiplayer game and i think sv_cheats has to be on