Another good suggestion and positive response to my Oregon search came from Shelley Hakanson at Wy'east Nordic Center. She says that on Mt. Hood "there is a very big contingent of folks who skijor." She described how much fun she has had doing it with her dog and encouraged me to contact the Oregon Nordic Club.
While she is definitely coming from the angle of ski joring with dogs, her e-mail gave me an idea. The fact that people keep telling me they have only heard about ski joring with dogs makes me wonder if I need to shift my focus. Maybe I should be contacting dog ski-jorers and see if any of them are breaking the mold and ski joring with horses?