Will a Canon bayonet lens hood fit on a Nikon lens?

And vice versa? Say if a Canon and a Nikon lens had the same filter size and both used bayonet hoods, would you be able to swap em out?

Probably not without major modification…

Unlikely.

Filter size is irrelevant. They may have the same internal diameter but external diameter will be different. I have Canon lenses with the same filter size but different lens hood sizes.

And the bayonets are probably different…

NO… Absolutely NOT. Bayonet hoods are a specialized fit for each specific lens. They are not even interchangeable between lenses of the same manufacturer, and most certainly not between different brands. The only hoods you can interchange are those that screw into the filter threads.