I work in a 275,000 sq. ft. rather complex building with total key card access (no metal keys accessible except through security and that's iffy because the lock-box with the key inside is behind a secure door - duh). Since power is out, I would break into the security office and get the metal key. I would then block open appropriate doors. I would either keep the key myself or designate a trusted coworker to be the keeper-of-the-key.
This, of course, in addition to the recommendations already described.
Note - I've been told some doors default to unlocked during a power failure and some don't. Maybe. I'm not counting on the tech who said that and I haven't seen the list of which doors would be unlocked.
Great topic.