Gordon Korman also falls into this category, though both of them have had relatively substantial crossover since.

I read the McDonald Hall books in the US during the mid-80s.* Do you think he's had more extra-Canadian exposure since then?

*I have to admit that I'm not sure many others did--I don't remember those books coming up as a common specimen of childhood reading in the same way as, for a sample, crummy Mercedes Lackey novels stand as a common specimen of early-adolescent reading, but that might just because most of the people I'd talk about books with would be fen.