Fog of War - Dynamic lightmap?

I’ve been bashing my head trying to figure out how to do a fog of war, where parts of the map that aren’t visible are half luminance.

The best idea I’ve had so far (this is all hypothetical at this point) is to generate a lightmap based basically on a top-down render of the units, but they’d be replaced with blurry white blobs (visible)…

I spose it wouldn’t need to be very large (64x64 would probably cover it), and it could update once every… 5 seconds?

Ideas? Thoughts? Would this be dreadfully slow? I’ve never made a dynamic texture, so… could be interesting

It seems a 64x64 is probably quite sufficient for my needs here.

This chap seems to have pulled it off - although I’m not sure about performance…

http://forum.unity3d.com/viewtopic.php?t=42387&highlight=fog+war

The project and demo files are at the bottom of the thread.

*EDIT - Sorry didnt realise this was in the iphone board, so it might not be correct!