I was reading through an old IML archive and saw that someone said that fog objects didn't work in global fog. Well I got news for you! It does! So I've made a little underwater scene with low rolling hills along the ocean floor and a submarine that appears from out of the fog. Ok so heres how you make a cool fog spotlight...
The axis should be sitting on the top of the cone.
The Y axis should now reach all the way down the cone.
And now you have a fog spot light to rival those of Lightwave. One thing to note though is that you must keep the wide end of the object hidden in what ever you're lighting up. eg. if you are pointing it at the ground, scale it so that the wide end is underground. You can make another one which has a closed bottom but it doesn't look rounded like this one. Render two, side by side and find out.
Well, I can tell you why you think the fog object is working and that's because you have the ghost texture added to it, but I can tell you if you take this texture off the object it will not render the fog thicker for the object. In fact even with the Ghost texture it still doesn't look completly correct at the edges. Try using it with a thick global fog and you will see what I'm talking about.
It does produce an effect similar to what one would expect, and in a light fog would probably work just fine. But I wouldn't go so far as to say fog objects do render correctly. Try a fog object with a nebula, fogtop or fogpaint textures and you will see that the fog object becomes clear in spots.
Anyways, I'm not knocking your fog spotlight. It probably works great. It looks familar to something I posted a tutorial on for 2.0 a long while back. But modified a bit for 3.0. Not that thats what you did. I'm just saying it looks familar, but then many effects do.
But Fog objects by themself still do not make the fog thicker.
But on the other side this can produce a nice shield effect for something.