If the DB is down, then ePO is effectively dead. I would strongly recommend that you take the ePO server offline, or at least stop the ePO services, while the db is unavailable. The client machines will still continue to run tasks and apply policies, and will cache their events until the ePO server is available again.
I have added some extra workflow to the upgrade process to shutdown the application server while the DB is unavailable.
Thanks for the response.