Not sure if it'd qualify for this wishlist, so apologies in advance if I'm mistaken.
Who would benefit
All wikis using the extension.
T69758: Identify extensions that need updates to work with user merging needs attention. If we can add support for at least the extensions in use at WMF, it'd be a big step forward. I think it is doable because it is not like rewritting the whole extension from scratch, but to identify specific parts of the extensions to work with this one. If we can make this happen, T49918: Rename of global (attached) users to existing global usernames would be achievable as we did with GlobalRename