EndNote doesn't do a great job when it comes to exporting XML, but I think I have a plan for processing what it *does* provide.
In EndNote Preferences > Reference Types > Modify Reference Types... there is a table that shows how each reference type maps field names to data. For example, for "Journal Article" we get the same fields down the left hand side but the right hand side contains the specific mappings for those field names. E.g.
While Journal Article uses the "Secondary Title" field to store the Journal title, the Book Section ref type maps it to Book title.
Books have Secondary Author mapped to Series Editor, but Book Sections have Secondary Author mapped to Editor.
So, it might take a while to create an intelligent set up in xslt to handle this, but it can be done bit-by-bit. In the four libraries I have so far there are only 3 different reference types: journal article, book and book section. I don't imagine that list will grow too long...