Well, this would be quite restrictive to gameplay and level design... but you could try just having a low ceiling :P
I don't think it's possible to set the angle of a camera with CS, perhaps cut-away view instead?
Now I'm not entirely sure of this, but what if you tried applying two camera scanners? Kinda like multiple FOFs? Chances are that'd just crash the game... and maybe your computer... but if you're desperate it's an idea. <.<
You could just set the camera to look up high, and enable vertical mouselooking... Wouldn't that work(ish)?