It looks like Ogg Vorbis has theoretical support in the spec for something called "Bitrate Peeling"[1], but there is no functional implementation for this yet, and there's been an open bounty on it since 2004.[2]
This is a really neat idea though. Truncating the packet to change the bitrate per client without re-encoding.
Peeling has been a goal for many audio/video codecs in the past. Nobody's been able to make it work acceptably, though -- either the low-bitrate version sounds awful, or the high-bitrate version increases in size to the point that it might as well just have a low-bitrate version alongside it.
So in that case you could peel off whichever stream you don’t need, but you waste some upload bandwidth over simply negotiating a bitrate in advance.