Maybe this is the rare occasion where big corporates can learn from Public/Government organisations. I was working for a UK.gov facility about 10 years ago, and they deliberately failed over to their backup generators once a fortnight for approx 20 minutes to ensure everything worked properly.

If the DC has a proper setup, I see no reason why they couldn't failover a small section to UPS/Generator backup once every month or two to check all is in order.........rotate through every section over the course of a day/week maybe