AFAIK Blizzard patched the Hinterlands thingy (lowbie tagging, high level out of group killing) as Asmon was doing it once; did not test it though.

Just do normal boosting; put 4 toons + 1 high into a group and then run through the level appropriate instances.
Like 1 run in Deadmines with 4x 20 + 1 60 is about 40% XP on clear... in maybe 10 minutes.