When a username is mentioned in a thread the user can not see, the mention should be rendered with a disabled style. The plumbing for this should already be implemented, and this should mostly require calling `setContextObject()` on `RemarkupEngine`. See T4411 and D11049 for the implementation in Maniphest.
The goal is to show the thread participants that a user can't see the thread, so they don't expect the user to respond or receive a notification.