A lookup table would be interesting. I considered that, but wavered -- would it be filled at write-time, or by some sort of memoization? Keep in mind that if it's memoized, you have to worry about when you tell the function what the plural should be, or you could get inconsistent data.
Is there a use case where you're not aware of the exception when calling the function?
I had imagined that the table would just be defined in advance from a src file. That would be consistent with other comparable features arc offers. And the memoization would just be a code efficiency thing.
If we don't care about being consistent, it certainly would be cool to have the table load from a data file, and then to have the first run of an override write to file when it's not present. Memoization to, again, cover code efficiency.
As for use cases I only had a few plural overrides I needed to deal with and I too just hard coded them. At the time I did so I remember thinking I should put it in a table, but never got around to it. Then I saw your post and I thought of it again!
Note that I don't really use arc these days, but I have ported most arc functions over to clojure and I'm always looking to improve my library. In my clojure world I would just store them in datomic as facts. I think I might run along and do this now that I've spent some time thinking about it ... ;)