Ability to mark too-late unregistration as 'valid reason'

One-sentence description

The ability to mark too-late unregistration as 'valid reason'

Motivation

Currently there is a separate sheet where we register the fines for absence at activities or unregistering after the deadline. Sometimes people have a valid reason for this. It would be nice to mark this on the website as well, so a separate administration is unnecessary.

Desired functionality

For people that unregistered too late or were not present during an activity, it should be able to mark their absence as 'valid reason'.

Suggested implementation

A checkbox or something?