Some exist already, you may gain inspiration from them. Project Wonderful is one I know about. There was some guy on here a year ago claiming to have an ethical ad network, but when I visited his client sites, it was mostly "Click Here to win a Free iPod" scams. I think you're right, it'd take a lot of work to find both clients and advertisers interested in working with you given your ethics-inspired limitations.

Edit: The other problem you'll run into is the tragedy issue I mentioned. Your ads will be blocked by most ad-blockers' scorched-Earth policy, which comes from your shitty, unethical competitors. Good luck to ya ;)