Some years ago, I worked on a patch to be able to silence all calendar notification of a given account. My use case was that I had a share calendar with my wife but didn’t want the notification of her calendar. I didn’t finish it since I was not able to convince Jolla enough for inclusion.
Your case could use part of the patch with a bit of glue to detect the out of office status of the account and silence all calendar notifications for this account automatically.
That’s adding another use case to mine. I may restart working on this feature…