I'm going to mix altruism with self serving-ism.

Does anyone know of any NGOs / charities / support organisations that operate in the Bangkok area who would offer some kind of visa sponsorship in exchange for (free/paid) work. I'd like to stay in Bangkok for a year or so, and this might be a worthwhile way to spend some of my time.

I can go the teaching english route, I have a masters and TEFL, but I'm going to see if there is NGO work.

I seriously doubt you would find any organization that would be willing to do that unless you have a very unique skill set. Good luck though. I might now have to eat my words. Look up www.reliefweb.int , look under countries and you will find an org looking for volunteer teachers. Hope this helps.