The 2009 estimate for total Laos population is 6.8 million. The Hmong-Laos population - based on what I can find on net search - is estimated 300,000. Hmong makes up about (rounded up) 5% of the total Laos population. This is probably similar to pre-1970s as Hmong are just another small ethnic group in all of Southeast Asia and China.
I find it hard that France would even consider giving the full government power to a small ethnic group that made up 5% of Laos that can barely sustain on their own. Everything the Hmong reaped in Laos - autonomy, Royal Lao political position, education, technology - was all given to by Laos. In other words, there was nothing the Hmong had that the French would've wanted so bad that they would give Hmong the royal throne. Unless the French really liked boiled mustard greens and pork ribs and Hmong pants with no penis holes or zippers in them.