Howdy Dan,
Welcome to our support forums and thanks for reaching out to us. I’ll help you out on this one!
The iCal importer won’t do the authentication, you’ll need to introduce the URL to the feed itself. Can you investigate if there’s a way to send login credentials or a private hash or some other way to authenticate trough URL, like http://user:[email protected]/cal.ics? Google Calendar for example lets you create a private URL to sync with other calendars. I’m not familiar with how OwnCloud works.
Please let me know about this,
Have a great weekend,
Nico