I've done a little modifying of PocketFrog and I've got my alpha blending working. What I did, as opposed to Max, was to add an overloaded blit method that will accept 2 surfaces. Currently, I use the second surface for my per-pixel alpha, but I was thinking it may be useful for something else.
I suppose it could be set up like the custom blit code to accept a user defined pixel function. I really don't know what all you could do with blitting from 2 images, but I though I'd mention it.
Ask if you're interested and maybe I can post code, but it's not simple since I had to modify several files to get it to work.