Hello,
I use S3 on few zenphoto galleries - but it's 1.1.2 version and within few days I am going to rewrite it to be compatible with 1.1.7 and later.
But first I have to find out how can I make it a zenphoto plugin, because my current S3 use is based on strong code modifications.
And it's limited only to be a copy of /cache/ folder - so all original files remain on my regular host, and only scaled, and resized images are served from S3.
Once I do it, I will share my code here.