What about a private blacklist maintained by the print exchange coordinators? You could ask for a reason from the non-contributing participant which is then put to a poll for the people who were participating in the exchange. If a certain percentage decide that the reason is not acceptable the non-contributing participant could be added to the blacklist and excluded for a certain number of months/years from taking part in print exchanges. I know I would be disappointed if I put the effort into making a print for an exchange and didn't receive one in return however there will be cases where the reason is legitimate (eg cyclones are wreaking havoc here at the moment).

Just a thought,