Instead of operating this as a charity (doesn't scale), why not sell the debt back to the original lendees at face value. (Buy back your $15k loan for $500.)
Personally I think they should try to recoup the face value from the debtor. If my debt was among the debt bought by OWS and they contacted me giving me the option to pay it off at face value so they could move forward with helping others, I'd pay it off in a second.
Actually I am. In fact, you'd be amazed at how many people are in this kind of debt who has learned their lesson and would love to be able to get out the debt once and for all.
I guess that wouldn't be legal, though?