Is this possible? I would like my shader to know how far each vertex is from the camera, to adjust the strength of an effect. Passing the vec_dist (from entity to camera) to the shader could work for some situations, but doesn't seem as versatile especially for large objects. Any thoughts?

After you multiply the InPos data with matWorldViewProj, the result (OutPos) will have the distance to the screen in its z-component (I think the w-component is some distance related information, too, maybe it is scaled (non-linearly) compared to z, I don't know).

Brilliant, good sir. Thanks, I'll see what I can do with that.