Thanks for your quick answer.
Let me use the WP core as an example. Imagine how hard would it be to create new plugins without WP's filters, and pay attention to the fact that there are filters everywhere in the core, even in the strangest functions, although the core developers didn't know if someone would use them. They do it to allow us -3rd party plugin developers- to interact with the core.
Add the filter at this moment would help only one developer, but you don't know how would it help others in the future. And it only requires to add the line I suggest just before call the api with the attachment.
I searched through the WP core trying to figure how to do it inside MudSlideShow without having to ask for your help, but the 'thumbnail' functions requieres the image to be in the local server, and MudSlideShow uses the images in Picasa or Flickr. As you have suggested I can modify the code inside WPBook, but this would require me to modify it on every new release.
Add this filter would help me at this mommnent, but would come handdy when other users ask you how to interact with WPBook.