I am deploying Mediawiki into an environment with a very old LDAP configuration. It uses posixGroup and posixAccount structural classes and the users and groups are not related by any sort of mapping other than the gidNumber. The groups do not have a memberUid mapping either.
I looked at using the UserGroupsRequest::Configurable but because I don't have a one-to-many mapping of any sorts I couldn't see a way to get this working.
When I looked into writing a custom group provider and saw how simple it was I just went ahead and implemented a UserGroupsRequest::UserGidNumber class. I am about to create a Gerrit commit for this but I wanted a ticket to attach it to.