I agree, tool names do not belong in directory names like that. It should be possible to move backends without changing installed file locations. Something like extra seems fine.
It’s currently the case and probably a good convention to keep, but it’s not necessary for any technical reason.
As an example of non-metadata that could be put there, one proposal I’ve seen for addressing the problem of where to install header files in a way that they become discoverable (xref problem description) is to use .dist-info/include, so build tools can be taught to look for them in a consistent place. There are probably better solutions for the header problem specifically, but not all other files that need a place to go inside a Python environment and outside the import tree of the package are metadata.
So to keep the options open, best not to use “metadata” as part of the directory name. It’s either superfluous (if everything is metadata by convention anyway) or limiting. I kinda liked [additional-files]; something like [distribution.info], [distribution.info-files] or [dist-info.files] all sounds reasonable as well.