By (at least US) law doesn't there need to be a warning before adult content is displayed at all? I don't see why this needs to be part of the short url.
Just go short url -> adult page. The adult page will check cookies or sessions or w/e and if none is found, display the warning page, otherwise it will show the content.
I'm not sure why you need what you are asking for.
For tiny url, I am also pretty sure that you can just put a "+" at the end of the url and it will show a preview page first.http://bitly.com/HUyAAk
+ instead of http://bitly.com/HUyAAk
gives a description of the page instead of automatically redirecting.