Lotus Notes 8.5.1: Undo Declining a Meeting?

IBM have a tips document that describe how to do this.

From the calendar Views panel, under Lists select Entries and Notices. Then you should be able to find the declined meeting request. Sort by an appropriate column to find the request (Last Modified worked for me since it was a recent request).

Once you've found the declined request you can open it and accept it.

If you open the declined meeting from the 'all calendar entries' view, you can choose to accept it, cancelling the decline.

Off the top of my head, I don't think there is a way. Perhaps ask the meeting organiser to remove you from the schedule, and add you again? Or just manually book the time on your side, and use a post-it on your monitor to remind you of the meeting :p