Let's say I have a simple mesh which is simply a flat rectangle.

Is there any kind of theoretical setting or property that can be applied to the mesh to make it only visible from certain angles?

Let's say I only want it to be visible of view from straight ahead, or a maximum of 45 degrees to either side. Is there any non-scripting method of achieving this effect?

Is it just me? If you have a flat surface staring at you, and you move to the side of it, it does disappear from view.... well, if it's standing vertical.

The nifskope documentation seems fairly sparse on explaining what each property does so I really have no definitive answer for you, but as a guess, possibly a billboard node mesh that constrains tracking to a certain angle?

Hmmm... that's an idea worth investigating. You're right, there isn't much documentation on it but maybe I could make something useful out of a billboard.