I really like the way that public service announcements show up in Google Adsense boxes when there isn't an appropriate paying advertiser to display there.
I think there is something you can do to really expand on this concept and offer sites that wouldn't be interested in displaying Google ads the ability to display public service announcements. For example, government sites and universities would probably not want to display Google ads but would probably be happy to display contextually relevant web service announcements.
So my suggestion is to launch a program specifically for those sorts of sites that don't want to display ads to only display public service announcements. You could call that Google PSAWords and you could give organizations that have public service announcements on them the ability to load in their own public service announcements in addition to the ones you already have.