I think it could be a easy implementation in stock code if a custom island insert meets your needs. Meaning display a ad between the first and second post of a forum.
Blue Man Group There is no such thing as stupid questions. Just stupid answers
And you can do it elsewhere with some modifications ... I have an ad island on one of my sites that only shows for guests, I also posted a mod "ubb nag screens" at one point that adds a custom island only viewable to guests