How does the user tries to connect / to create the calendar?
According to <
http://www.horde.org/apps/nag/docs/CHANGES> Nag 2.1 added
the feature "Add ics.php, which can be used for integration with
Sunbird, iCal, et. al." Maybe you can play with this ics.php URL and
setup an iCalendar based remote calendar that links to this URL?
According to <
http://www.horde.org/apps/nag/roadmap/> Nag 4.1 will
contain the feature "CalDAV server capabilities". In my opinion it means
that Lightning should be able to talk as CalDAV client to Nag once this
new version is released and deployed on the server.
Stefan