Thanks for the request. Currently there is a way to set this property through powershell for groups, but it only works for users in OWA. We have this in our plans for Outlook desktop as well and will be working on it.
Here is what we need. Member A puts in a OOF - PTO day, only Member A can change or delete said PTO day. Right now anyone can go in and delete someone's PTO day. The Appointment should be readable by anyone but not editable unless its an owner. Owners should be able to cleanup calendar events.