A guy sits on a woman's face so she has to lick his ass; or he rubs his ass on her face. A definite sub-variety of ass-licking, but quite hard to find?
#3
http://gal.monstermoviepass.com/mov/acidrain/stackemdeep/01/
#2
http://hosted.el-dvd.com/g/x/track/2737/movie/68/40939
#2...