I too would suggest a method of blurring like mentioned above. I have done something similar to this in my work in which I wanted the average color of a texture so I could use that color to bake in vertex coloring.
The way I went about it was to scale the texture down to 1x1. Of course, you could achieve a little more color resolution by not shrinking so small, but the point is that by scaling the texture with some sort of filtering gives you averaged samples.
I automated this process using MEL and the external command line tools ImageMagick (www.imagemagick.org) for the scaling of the image. The process can get a little convoluted, so your success may depend on your comfort level with MEL.
--JeffD