This, so far, is a hypothetical situation, so don't put yourselves out to answer.

Consider the following circumstances:

1) a dual national (Thai, and, say British) enters Thailand on the foreign passport, not having a Thai passport

(very common occurrence);

2) loses his non-Thai nationality while in Thailand (rare, but not impossible);

3) and is listed by the US as a probable terrorist and therefore not to be trusted on an aircraft.

Can he change from stateless to Thai by getting a passport and then doing a land-border run? I had thought he could, but another post suggest that changing active nationality to Thai by doing a land-border run is not allowed. I presume he could not get extensions of stay in Thailand on a cancelled passport!

As you say Hypothetical and not worth the Headache,the crux of the matter is: you can't second guess either Thailand or the UKs take on this unless you ask them.

Can he change from stateless to Thai by getting a passport and then doing a land-border run?

A dual Thai/British national who loses his British nationality will not be stateless; he will remain a Thai national.

USELESS DRIBBLE!!!

pass

No fly list is for flights to the US and flights with US-airlines, it is AFAIK not for flights to neighbouring countries with a regional low budget carrier.

A person with Thai nationality entering on a foreign passport is subject to immigration control, but can not be thrown out. He would jus show is Thai ID-card or passport if police wants to check. When leaving the country, he would pay a fine of 500 baht per day with a maximum of 20,000 for being on overstay.