if, for example, the named user is on vacation or is absent due to sickness
Woah! I had to go an ly down for a moment when I read that.
That is actually a major problem out in the wild. I had no idea is was acceptable from a licensing perspective. Holiday is not an exceptional situation though and with SSO there is no password anyway.
There are better solutions such as substitution management for this. No reason to break audit trails for who did what in the system.
Cheers,
Julius