PVRTC 2 bit Lightmaps

I'm trying to cut down my app size and I noticed that Unity won't allow me to use 2bit compression on textures marked as normal maps or light maps.

I found a work around for normal maps, as long as I don't mark the texture as a normal map then I can just do some extra work in my shaders instead of using the built in function in UnityCG.inc to decode normal maps.

I tried the same approach with lightmaps but I don't get the same lighting. I'm wondering how Unity modifies a lightmap texture when it's marked as a light map. What code do I need to put in my shader so that the lighting turns out the same?

thanks for your help.