I am still seeing no research done on this. This is what that website says...
"Tobiano is probably the best recognized white pattern. It is a dominant characteristic, meaning that if only one parent carries tobiano the foal will have a 50% change of inheriting it. It tends to affect the legs, shoulders and hips, and leave the barrel of the horse colored. It can cross the top line and where it does, it will cross at the hip and shoulder first. Tobiano alone is thought to leave the face solid, but very few horses carry tobiano without having one of the other white patterns as well."
From what I have read from that website. It is THOUGHT to leave the face solid...if this was proven true they wouldn't have added that. If this were true then every single breed that has some form of face marking carries an overo gene. It just doesn't work.
I've spent most my life riding horses...the rest i've just wasted.