Hmmm, I don't think I've heard of anyone doing this and honestly I'm not sure how I feel about it. A lot of artists, myself included, have ad spaces on our comic websites through Project Wonderful or similar sites. But it sounds like you want to pay someone to endorse your comic and share it with their readers?
I feel like if someone asked me to do that I'd say no, unless I already read the comic and really liked it. Then I'd probably promo it for free, as I've done in the past with comics I enjoy... but even then I don't think I'd post on my Tapastic page, I'd probably share it on my twitter, blog, or somewhere on my main comic site. I like to keep my Tapastic just for comics.