I don’t think you can use the on Mikrotik webserver because you cannot redirect through URL, My workaround is partially working because I don’t know why I cannot get Mikrotik mark the packets I want through the Mikrotik-Mark-Id, please view my other post here http://forum.mikrotik.com/t/radius-attribute-mikrotik-mark-id/26594/1 if you also can help me 
, that would be great …
My workaround was to set up a IP/port Virtual host in Apache and changed the default page to the one I wanted to show to my “special” customers, so I just dst-nat the customers’ traffic I want to my webserver’s IP address and to this specific port …
Hope it helps,