I have the same problem.....I am Belgian residing in Thailand, and stuck in South-Africa for the moment....

There must be a way around, because the system is not very fair. Existing medical conditions are in most cases excluded by the 12 approved approved Thai insurance carriers. What can you do with such an insurance in case the biggest part of your medical costs in Thailand are not covered? That's not what the Thai government had in mind I guess. There is a possibility to use also other private companies, as long as the foreseen form for this case is signed. In my case, I have a nearly 100% coverage without any limits, but not from a private company provider, so that doesn't help me. So I am exploring the possibility to change my Non OA visa into an Non O visa as you indeed mentions below.