I've considered it. I wear a custom guard for hockey. It's an added bonus for concussion mitigation. I think the chocking matter isn't an issue. They are simply too large in comparison to the cast quantity of water that comprises a major choking hazard. I would imagine a full face is the most effective with a mouth guard being somewhat moot. Though, even the best full face can't keep out that one pointy rock!
On the river, I can abandon who I am and what I've done. However brief it lasts, while on the river I am nothing important and everything insignificant. I am flotsam, and happy to be so.