> reconfirm those bits afterwards

Thanks - I hadn't thought about that but it seems to be the main downside of this approach. The benefit of NaN-boxing is that it reassigns values that are otherwise unused - floating-point calculations will never generate NaNs with those bit patterns.

An additional wrinkle is that NaNs are a bit unstable and can have large performance penalties. You can't let the NaNs ever escape into arithmetic and you may even have issues even storing them in a register.