I worked this out a while ago; only lately did I think it could be used this way. I have seen a few occasions (not recently though) where people have asked about a "one way" idea, where a player can only travel in one direction, but is blocked if they try to turn back. Ok, here it is;

1. Draw out a small brush, (larger than an actor), and change the 2d view to side on (so that the brush will stay at it's current size at the next step)
2. Go to patch>simple terrain mesh OR simple patch mesh
3. When the dialogue box appears, choose any density, it is not important.
4. You now have a mesh that is transparent on one side, and textured with whatever you choose on the other. Rotate the patch by going to selection>rotate>z - this rotates the mesh on the Z axis by 90 degrees, or you can use the Abitrary Rotation tool.
5. This list shows the properties of the two sides of the mesh;

Textured side;
Clips; player, items, grenades, rockets, bullets, etc. This side could be made transparent (but not with caulk, this doesn't work properly) with a custom texture, so that it is invisible too.

Non-textured side; (Shows up as wireframe in the editor) clips nothing, bullets go through, and player can pass through.

Bear in mind these two things;
Shadows - if a mesh that casts a shadow is invisble, and you don't want it to be found...well, you'll have to do something about that.
Texture mixing - if the mesh is part of a wall, then it isn't that easy to make it's texturing blend in with the rest of the wall, you will probably have to manually rotate the texture (right_alt and right click drag) or move it (left_alt and right click drag)

I think that covers everything...