I agree that taking the bags with you is the best option. I never leave my panniers on my bike when I lock it up, not even if it's just 10 minutes.

However, if you're committed to leaving the bags on the bike, Ortlieb makes cable locks for their panniers; maybe you could find a way to modify them to fit your bags.

http://www.ortliebusa.com/CartGenie/prod-136.htm