Hmm, I should really read and think before I post..

I hadn't seen the pow() at all. And about that project.. Can't seem to find it

I still remember some things that might be usefull for improving this effect. You can compute a luminance value by dotting the original color with a precomputed luminance vector like this:
float Luminance = dot(OriginalColor, float3(0.299f, 0.587f, 0.114f));
You can output that value in the first pass and then use several blur passes to blur the result and then blend it with the original image. This way it can be made to work for 1.1 shaders, but you could also do a single (gaussian) blur pass.
This also allows you to controll the way you blend the blurred and original image; when leaving a tunnel/cave, you could create an 'overexposure' effect by turning up a variable in cscript.

Not sure you where even looking for advice, but here it is anyway