And as the phrase "reserved" in the error message indicates, it will likely be given a meaning once all the ramifications of doing so are worked out and the best choice of meaning is decided upon. If you're impatient and don't want to wait for that, define it to do what you want. Your code won't even break when it is given an official behavior since your method will overwrite the built-in one.
But this is exactly the kind of thing I called a bothersome corner case. Needing to redefine a global function in order to use fairly intuitive behavior is not great developer experience.
When things like this are left undefined, it's not to intentionally annoy you, as you seem to be taking it. It's generally because there are two or more reasonable possible behaviors and which one is correct hasn't yet been determined. In this particular case, there are subtleties because named tuples can be seen as ordered collections of values and as named associative structures. Deciding that kind of thing takes a lot of time and effort. If you feel that there is a preferred behavior that broadcasting over named tuples ought to have, it would be helpful to post that on GitHub.
