Well, your idea is good. I am also planning to use YPN. For now, I'm only using adsense and it gives me good revenue.
YPN signups are by "invitation". You can fill the invitation request form here http://advision.webevents.yahoo.com/ypn/leads/index.php?loc=USYPN0005
I have Registered the Yahoo Publisher Program, But Didn't get the business Id Yet. If it require long time.