Ability to check if a username is available as an admin

[Half-Shot]

As part of a project I'm working on, I'd like to see if a given localpart is available in synapse without having enable registration. The details are a bit complex but:

• We don't want to enable registration because we're using SSO (and don't want users to simply stroll up and register an account directly).
• We have appservices on the host, which means we cannot simply use the SSO provider as a means to check if a localpart is avaliable. Synapse is the best place to be checking if a username is available because it's aware of these.

Essentially, I'd like for admins to still be able to call https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available regardless of the config setting.