So after a little more research, it looks like crossing China might be a little difficult to do and with rules dictating a local Chinese 'minder' be in attendance at all times it doesn't bode well for the original idea. Some investigations a reports from people who have undertaken similar adventures, seems to suggest that crossing Kazakhstan, into Tajikistan, a brief entry to China before using the Karakoram highway into Pakistan, India and Nepal. Where air freighting the bikes from Katmandu over the blockade at Mayanmar(Burma) to Bangkok would be the best route to attempt.
Overland routes map
Once clear of that obstacle then we can resume our route with time to explore Cambodia and some of Vietnam before heading south to Indonesia.
Also in plan now is to visit the Horizon Unlimited Motorcycle travellers meeting weekend in July 2012, where lots of useful information, ideas and suggestions can be explored.