Especially on windows, this would be very useful. It would also solve the request in thread 1713638, the compatibility to other clients. You would just have to select such a virtual device in Teamspeak, Ventrilo, Skype etc. And I don't think, that this could harm any licenses, because it would just be a redirection of audio streams, all these non-free softwares wouldn't have to be reverse engineered or touched in any way.
You should be able to stick a virtual device on a whole channel (with or without your own voice input) or on certain people. That would give great possibilities in redirecting or recording the audio streams.
Maybe some people know the Singstar clone "Ultrastar", which is available on sourceforge as well. Because of the low latencies Mumble provides, you could also be able to play this game over the internet/intranet.
If a feature like this doesn't fit in your policy either and if I'm just spaming up this forum, just tell me :P.
Instead of adding them to the server, making a virtual client for these purposes would be better. Anyone with the programming knowledge can take the mumble client source and graft on whatever would be needed to attach to a teamspeak channel, or a ventrillo channel, or be a sip client and run it separately on the same server as their murmur server 9or even a different server if desired). This would also take better advantage of threading on newer multi-processor systems, and only the resources of those gateways / virtual mumble clients the server admin wanted would be used instead of bloating the murmer software codebase.