Hi. It is impossible to do or very hard to do to redirect to alert site.
The problem is that when you detect that site is harmful connection is being established between client and harmful site. Bat where is no connection between client and aler site. If you redirect such packet to alert site it simply drop it.
The one easy way to do it is to answer on GET query packet by yourself. Just create packet with answer as any site do.
But it needs some some special knowledges about tcp and http.
Regards,
Andrew