It would be useful to receive a Deleted (or other keyword) notification when a Resource rejects a meeting invite. This notification would need to include the date/time of the meeting request and the organizer ID since there is no corresponding event object to query.
This is useful in conjunction with Outlook Add-Ins that may have done some work in the Organizer calendar with the expectation that the resource(s) will be reserved but currently there is no mechanism to know that the resource request(s) were in fact denied.