There isn't a way to do this with shared calendars via CalDAV in the current public release, but I think this should work better in the next release as we've introduced a number of sharing improvements in a variety of areas from webmail to protocols. I'll do some testing there to see if I can confirm that for you and I'll let you know what I find out. If you want to try the new changes out for yourself please reach out to our sales team about the beta program and they'll be happy to get you set up with a trial key for a test server as well!