Have the trick or treat debuff on you
Join a random heroic
Right click portrait and leave group IMMEDIATELY if it isn't the random you want
And to sweeten the deal, I found a way to reset the random dungeon finder cooldown, just queue up for a Headless Horseman run, take the 2 minutes to down him, and it resets.
Leaving a Headless Horsemen run with the Trick or Treat debuff will give you the deserter debuff.
I'm using this personally to try to get more oculus randoms, gotta get that drake
Screenshots:
http://img178.imageshack.us/img178/8...2210194937.jpg
http://img811.imageshack.us/img811/7...2210194945.jpg
Edit: Found a way to reset dungeon finder cooldown
Edit 2: Apparantly you need to leave very quickly or you get the debuff
Edit 3: Screenshots added