Rob, I think most people understand that starting a union would be more trouble than it's worth, however the idea of some kind of professional association that can offer support to members (legal advice, professional advancement, resettlement support, guidance etc.)is, although difficult to establish, a good idea.
Your comments in the last paragraph go to show why Blackmail's idea could not work, too many people just don't give a fig about anyone else and are quite happy to shaft fellow teachers............I'm alright Jack!
Clearly you have had wonderful employers during your time in Thailand, that is not always the case, your statement that if "You do a good job, support the people that support you and the question of unions becomes unimportant" is complete bollocks, in many schools it matters little how good a job you do, and you'll often find that the people that you "supported" are the first ones to put the knife in your back.