Belgium embassy doesn't issue these letters, they write it on their website ( you still can explain them your situation and see what they say, but I wouldn't count on it) , so I guess the only option you have is to go for the yearly extension like Tod said.
However there have been people who got told by immigration officers that they are under the visa amnesty when they went to apply for a 60 day family exstention. Contact your immigration office and ask them.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.