Looking at the OS maps I have of the area (10 years old), there's not even an unmetalled track for the second half of that route: sheep trails only. I would expect some reasonable amount of big-based hike a bike. The main road is pretty quiet!
Thanks for checking and sharing. I'm thinking of steering clear of this, then. Not sure how hike-a-bike will turn out with the panniers, and I'm really afraid of a big mechanical while in the middle of what appears to be nowhere proper...
looks like a tiny hiking path to me. I'll cross my fingers that someone here knows about it and can recommend either way