Gaurantee advance booking for soft sleeper from Chengdu - Lhasa

We have a unique situation. We are in the process of arranging a multiple country trip using airline frequency flier award miles. The available seats are extremely limited and need to be book 8-10 months in advance. Making changes to the flight schedule within a month/couple weeks is not possible - no award seats. The China rail system make tickets available within a couple weeks/days before departure. We are wanting to book soft sleepers from Chengdu to Lhasa, and connect to one of your tours in Lhasa, and connect to a flight from Lhasa to the next non-China destination. We do not want to be in the situation (1-2 day train travel window) where the train is sold out for non-Chinese travelers or there is only hard seats available for the 30 hour journey. The travel to this areas would be late June 2014. Are there a limited number of soft sleeper seats made available for advance booking situations like this or for organized China tours from Chengdu-Lhasa? Thanks