I would start by creating a share (Manager) from the conference room's calendar to the owner's account, then mount the share in Zimbra Web Client. This should at least cause the owner's iCal to show the room's calendar. (You might have to refresh iCal before this will happen.)
You
might then get the accept/decline.
Another method that seems more promising in theory would be to share the calendar and then, in iCal, use Preferences>Accounts>Delegation to access the conference room's calendar. However I haven't been able to access shared calendars this way, see
Bug 45853 - ical delegation: iCal can't alter settings or access shared calendars