Dunno, but I have a concern.
Firewalls rely on knowing where traffic is going.
A MUXed protocol is likely to require inspection of every byte to do this, or at least, reconstruction of the stream, if it is to be noticably different from using multiple connections. This will make firewalling almost impossible in hardware, and resource-intensive in all circumstances. In turn, this means that it is likely to be banned from most applications, which means that doing it is a waste of time.