Does anyone know of any good routes through the Eastern himalays ie. from Bangladesh to Hong Kong... or for that matter any routes through the Himalays. How accessible is it? Are the roads on maps viable?

There are lots of potential cycling routes in the Indian Himalaya, just do a google search or check the Adventure Cycling Handbook. https://www.adventurecycle-touringhandbook.com/

The Eastern Hill states of India have seen a lot of unrest and travel is often restricted.

As far as getting to Hong Kong, you may be able to cross the Nathu La in Sikkim from India to China. I don't know if the border is open to independent travellers by bicycle - check the Lonely Planet Thorn Tree forums.(edit-) as far as I can tell the border is only open to Sikkimese traders at present-not even Indian nationals can cross into China.

If you were thinking of going Bangladesh to Hong Kong via Myanmar -I don't think you can do it. There are very few land border crossings into Myanmar open to tourists - you can go from China into Myanmar along the "burma Road" but I don't think you can go the other direction.

(edit again) Further perusal of the Thorn Tree makes me think it may be possible to enter Myanmar from China and exit to Thailand. Maybe. If you are lucky.

My cousin and 3 other guys are making their way through Tibet as I type, somewhere between Ali and Lhasa. I did 500km with them through the deserts of Xinjiang (China).
My cousin's blog:
https://centralasiatotibet.blogspot.com/

These other blokes entered China from Pakistan to Kashgar.
Ferran started from his home in Andorra (city state between France and Spain) on his 20yr old trek and pulling a bob trailer. He has logged 20,000km already on this trip. His blog is in Catalan, and I read it by using a online page translating webpage.
https://ferranpedalant.blogspot.com/

Another is a Dutchman they metup with, used similar translating page to read
https://klimmertijn.blogspot.com/

That is a heck of a ride, I turned back because it was my first tour and my bike was not fitted properly and my legs were already fried, before encountering the first major pass. Will attempt it next year with better fit bike and more touring/camping experience.

tiger474: Those are some awesome reports! Pictures tell a thousand stories! I will be watching your cousin's progress.