I think the technique of retroactively granting someone a special license should be used more often. You know how lawyers are always jumping up and down on people about trademarks because of "dilution" rules, even to the extent of pissing off supporters/fans of their product? Instead of sending out "cease and desist" notices, they should send out notices of permission, granting their supporters/fans the limited right to use their logos, etc. This way their trademark is not diluted ("they had permission, your honour") and the fans aren't peeved -- in fact they are probably chuffed.
Getting back on topic, if someone unintentionally violates your copyright license but you happen to like what they've done because it's in the right spirit of things, then grant them a special permission.