I guess it would be a question of likelihood versus possibility. Would it be likely to have found a fusil de chasse at a Rocky Mountain fur trapper's rendezvous? Probably not. Would an American mountain man have carried one? Even less likely. But would it be possible? I think so... A few more or less intact French fusils have survived into the 21st century, so there were probably even more of them floating around 200 years ago, and these guns did get around. A TVLLE marked lock plate was recovered from the Apalachicola River here in Florida a few years ago. Who knows how it got there... We had a few Frenchmen down this way in colonial times, but not very many. In any event, I don't think it would be too big of a stretch to imagine a French voyageur, a real hivernant, carrying his trusty old fusee to a trappers' gathering in the Rockies. After all, rendezvous is a French word, and we know some Frenchmen attended them. We recall Kit Carson's famous duel with "Shunar," which was a corruption of the French surname, Chouinard.
I'll still maintain that the American trappers preferred rifles, though. This is mentioned several times in the literature of the period.
Best regards,
Notchy Bob