It's not possible to limit visibility of channels on mumble at all.
BUT:
Mumble is able to run multiple virtual servers inside the physical host process relatively easily and can be managed from a single location (our authenticator script). We're using it at the moment to provide Mumble access to waffe on their own virtual server instance, and it's not particularly difficult to add new virtual servers to our mumble setup right now.
That said, the services scripts have not been updated yet to handle such wonderous techology and therefore modifications must be made directly in the database. If you can give Ander the name / ID of the forum group you're using to control these friends, he can add it as a bitmask for services, so I can set it up to limit access to that virtual server (i.e. so only people in that group can join the virtual server). Will require a little bit of setup but it's definitely possible if you want to go that route. Оnly rеal way to stop the visibility thing being an issue