Just found on Bact's blog a link to the Thai national standard TIS 1099-2535, which defines the two-digit geocodes for the provinces. This standard, published in 1992, is in fact identical with ISO 3166-2 (except that the ISO added a special code for Pattaya). This also explains why the ISO codes for the provinces did change between the draft standard of 1988 (which was based on the US created FIPS 10-4) and the final published one of 1998. I wish I could read the text on that website, it might give me even more insight on the origin of this standard.
There is also a more condense page which only lists the codes. The standard was also published in the Royal Gazette, in volume 109, issue 104 ง, page 9011, published on August 20 1992. That PDF also includes the 8 pages of the standard itself.