A new tool to export as EPUB has been created for Wikisource. It is called WSExport. It is running currently on the Toolserver (see discussion here). Probably it should become a Mediawiki extension or maybe share some base code with Collections 2. Is it anyone who could help to make an extension out of it the same way as happened with Extension:Proofread Page? The main developer says he has no time to do this right now.
Topic on Talk:Collection Extension 2/Flow
This would be great! I will help spread the word to try to find someone...