Volumetric Fog Help


I am currently working on rendering volumetric Fog.
Currently, I have it working using multiple passes.

First, everything in scene is rendered to a texture.
Next, I render the Fog’s (Cube) back faces, to store data into a new texture ( u, v, w, screen depth ).
Then I do render the front faces, passing the previous two textures into it.

My problem:

When you go inside the fog, the final pass does not render the whole cube do to some of the cube being behind the camera.
At the moment, I have a fog effect that simply uses screen depth, because when the data is lacking, I can assume it is 0.0, due to 0.0 being the start of the camera frustrum.

My question:
Is there a way, I can tell OpenGL “If the camera intersects this object, assume a new face is there, to fill the intersect, and accuratly interpolate the vertex data across it”.

This way, I can access the UVW as well as depth values and get some pretty sweet looking fog with a perlin noise 3D Texture.

Here’s hoping