Lived and worked out there for best part of 4 years. Never volunteered to do anything.
I know there are charities that deal with refugees on the Burmese border. I was talking with one years ago about taking over someones position there.
I'll dig through old emails and see if I can find their contact details.
remember you do really need to have a work permit to work as a volunteer out there. There were all sorts of problems after the tsunami with volunteers working.
When applying for your visa try using the Hull consulate and apply for a non-immigrant O or you might be able to apply for a non-immigrant B if you have a charity that will support you. It's harder work changing a tourist visa into non-b with work permit than having a non-b or non-o to start with and once out in asia it's a right faff to get a non-b or non-o.
The visa you get on arrival is just a 30 visa on entry and isn't changeable without hard work again.
If you find you have to stay on tourist visas for a while out there, you can (think you still can) back to back tourist visas which just require you to leave the country and go back in to reactivate them.