I might not be able to help you with an automated output using context, but I
can recommend a very good book on e-book creation. Elizabeth Castro (ISBN
978-0321734686) discusses a sample e-book line by line of the source code, how
to create it, validate it (http://code.google.com/p/epubcheck/) e
(Yesterday’s beta in suite=minimals, OSX-Intel)
With
\setupbackend[export=example.xml,xhtml=example.xhtml,css=example.css]
in my project’s environment,
I get both "example.xml" and "example.xhtml" (as well as "example-
images.css" and "example-style.css"), starting with the same content
(up
