I had this rattling around in my head and finally found it in an old FAQ.
Question: Are the mountain hexes north of the Ledo road part of Northern India or China?
Answer: They are not listed as part of Northern India, and if they are part of China, no one can enter, so they would be unplayable. On the other hand, one might be able to trace through them...
Not official, but compiled from ConSim World.