The Centurion lounge in Concourse C will let you in 3 hours before it closes at 9 pm, so that may be an answer if you are taking the late night flight again. You can get in with your Reserve card.

You didn't miss much in the DEN Sky Club though.

Both the time limit and cutting GM/PM/DM access have been short-sighted in my opinion. It's shown people that Delta's only real loyalty is to Amex.