I thought of a way to blend images that I haven't seen before. I wanted to see how it looks, so I implemented it in vvvv, and now I've wrapped it up as an FFGL plugin. I call the blend mode "color wrap". Here's what it looks like:
This is the first mixer effect that I've done (well, I did do a time switcher once, but that doesn't really mix the images :). FFGL mixers are basically just effects that require 2 input textures. The first FFGL parameter you define will be bound to the opacity slider in the host application (by convention, anyway).
Assembla repo, as usual.