I could definitely add the nature when I was pre-testing the plugin as an eclipse project before making the ui/doc plugin, and tested sysinfo and ij successfully (so the core plugin is ok).
Most likely something went wrong in the export step of creating the ui/doc plugin.
I don't think this should hold up the release - the ui and doc plugin have their own version scheme and do not actually tie into a specific version of the rest of the derby code.
(The core plugin is part of the full release, it includes derby.jar, derbytools.jar, derbynet.jar, derbyclient.jar. That part is working correctly).
I'll try to make a working ui/zip plugin today.