Since this attack is based on Router Advertisements, you need to be on the same LAN to exploit it. It also does not apply if the LAN implements RA Guard (RFC6105).
http://www.youtube.com/watch?v=8Q8EFwKVKdA for some non-trivial but ingenious ways you can get to a LAN from the outside. (Then again if you're as useless as my ISP, leaving the telnet server on the DSL modem with a default password, listening on the WAN, you don't need to do anything fancy to exploit LANs)