Dual pricing at government hospitals is probably more widespread than you are implying. Also, given that Thailand is home to millions of migrant workers from neighboring countries (including illegals), with estimates suggesting there could be in excess of 10 million foreigners from neighboring countries here, I think most hospitals throughout the country would have treated Lao, Burmese, Cambodian, Vietnamese etc. citizens at one point or another, and many do so on a regular basis. Hospitals out in the sticks are less likely to have treated farang, of course.
Also, being Thailand, greed sets in, so I'm sure a hospital that receives a farang patient would more than likely find an excuse to charge them more than locals...especially if it's a more involved case. For a simple wound cleaning or the sniffles, probably not.