I'm not 100% sure whether SAS has what you describe, but on a number of flights, I've seen one of the crew member go to the gate desk with a friend or kid and ask for seat assignment for their standby ticket.
Regarding taxes, I guess it's like many other benefits. You need to report it your self if it's over the limit for free benefits, and then we get back into the discussion about what is the value of a standby seat on a flight.