[keycloak-user] Cross Realm Administration

I've found a few list posts musing about cross realm administration but nothing concrete to say whether or not it's possible. So what's the status of it? To give some background on my use case, I have three kinds of users: staff, contract staff, and customers. It seems to me that a realm for each kind of user is the best way to model this so that way groups, roles, and login screens are specific for them. Certain staff users (admins or managers) need to be able to manage the contract and customer users like master realm admins can manage users in other realms. So how is this done? Or if it's not possible then should I use the master realm as the staff realm, or should I just have one realm and model it by way of roles and groups (still the issue of different login screens then)?