I do wonder, if its is a possibility that the use of fire was location specific. Marc, being in Canada as you are, the need for heat on a bow is probably less than the need for it in the humid areas of the east coast about from Virginia and south to Florida. Everywhere else as you say, you get more likely to break a bow than benefit from it. I'd say its probably related more to specific woods as well than related to all... say Hickory and not elm so much. That's again assuming it done at all.
That being said I have cooked a fresh cut stave, reduced to floor tiller, over a fire while camping. The next day that bow ( persimmon) was a very dried out fast shooter. It wasn't heated to the point of heat treat, just over the heat to the point you held you hand over the fire and it wouldn't burn but hurt real bad after a few seconds.
My dad told me stories of Indians using fire to make bows stronger. That was in the 90s, before the quick spread of information. One must wonder where everyone got the idea that natives did this. Honestly, I dont see natives stock pilling bow wood like we do. Chances are good, they cut a stave, fire cured it, then made the bow. We have never found a stockpile of roughed out bows set aside to cure much less a stave stash. Then of course, if the actually did store staves in ponds to keep bugs off, fire would probably be used to dry out the bow blank over a period of days....
Running the scenarios through my mind from all possible directions, it keeps going to the same spot, fire cured, not fire hardened for the sake of less work in cutting and storing staves, with the possibility of select locations heat treating select woods for humidity protection. But then again... tallow goes a long ways to prevent that problem as well.
