The current patch used longer than expected time to install; while I wasn't affected (GMT+8), I heard a lot of European players were.
When the servers went back up, OOO announced elevator fee reduction as a compensation. However, the European players had already went to bed.
My suggestion is simple: Record the hours the service was interrupted, and give the elevator fee reduction one day later at the same hours.
e.g.
Servers went down unexpectedly from 21:00 to 23:15 on 2012-05-11.
Elevator fee will be reduced from 21:00 to 23:15 on 2012-05-12.
Justly give the compensation to those who were affected.
Makes no difference really; there are too many unknowns.
That day could've been your last 'free' day in a while, and 'tomorrow' could be a school/work/something else day.
Is it possible to track who TRIED to log on? If it is, why not send something (mini mist tank or short-term elevator pass) to those who did? I think this'd be easier if it were possible, and is more likely to give back to those who were actually affected.