As it was recommended at the New Mandala blog, I got myself the book Creating Laos and am currently reading it. While obviously most of the content is off-topic for this blog, I have just came across an interesting bit on the first national census in Siam in the year 1904. For the author the main point is that there was no distinction between a Lao and a Thai ethnicity in this census, while Mon and Khmer were counted separately according to their language, and the Chinese according to the traditional pigtail hairstyle for men and the clothing for women. Those ethnic Chinese who already adopted Thai clothing or hairstyle were instead counted as Thai.
I did not know about this census before - both statoids.com as well as the underlying book start with the 1947 census, and also the National Statistics Office claims the 1909 census as being the first. This is probably due to the fact that in 1904 the census was only done in some parts of the country - central parts of Siam excluding Bangkok and the outlying areas of the Isan and the North (Lan Na), this it wasn't a nationwide census.
The whole section is based on the paper An early Thai census: translation and analysis by Volker Grabowsky. If someone could provide me with a scan of this 91 page long paper it'd be great.