For future reference: to allow validating, add the line
<!DOCTYPE book SYSTEM "http://www.megazine3.de/megazine2.dtd">
to the top of your book definition XML (.mz3 file). Otherwise validome won't know which doctype to use.
Also, to clarify Hans' statement regarding why this alternative syntax probably won't make it in: child nodes (and text is technically a child node) are used for several other things, such as dynamic scaling and localization. So I'd like to avoid possible hiccups arising from changing that bit :/
But as Hans said, just push your XML through validome and it'll point you to all those spots anyway