I requested a new collation for Norwegian because of a Wikipedia-specific usecase, and was told it would probably be best to override our collation order for Norwegian with a custom collation sequence in MediaWiki only.

That would actually be great, but we don't actually have any mechanism for doing that. When the ICU collation isn't suitable for our needs for some reason (like recently with Inari Sámi: T388998/T388970), the only recourse we have in MediaWiki is to create a custom uppercase collation, but that misses out on a lot of stuff from the ICU collations.

So ideally, we would be able to specify and use custom ICU-style collations directly in MediaWiki.