I am just designing our October Holiday Ride 2014: Kunming - ShangriLa - Kunming. 1700 km, 6 riding days. I believe this will also be a great ride. Lasha/Tibet is a dream, but there are also nice destinations without all the administrative trouble. a HD Softtail is definitely not the right choice of bike there. we will go on BMW bikes.

Dear Anguscharlton, open Google Maps. set your starting point, choose the roads, stay between 250/350 km/day, find an overnight place .................. and you can check if your idea is working.
Ok, on westerner/American Expressways a days distance with a HD ~ 600km is no problem, but here, with this road conditions,...... for 300 km you will need ~ >8 hours (if nothing breaks down).
it is also important which time of the year you will go on the trip. believe me, you will not be happy riding a Chinese countryside road in the dark or snow.

last year OHR we went from Suzhou to Jingdezhen and back. the yellow G-Roads have been, mostly, in terrible conditions. under construction, gravel, potholes, heavy truck traffic .......... a HD is the wrong choice of bike for this conditions