"It also allows the cancellation of existing remote port-forwardings using -KR[bind_address:]port."

~# first of all shows really ugly information :-)

Shell doesn't work with muxing connections. However in 5.4 forwardings were supported with it, but it means you still can't remove local/remote forwarding without killing the master connection after they've been established (really annoying with controlpersist).

If I'm wrong on this please let me know, I'd love to be able to know how to get around it.

http://www.openssh.org/txt/release-5.4
 * Rewrite the ssh(1) multiplexing support to support non-blocking
   operation of the mux master, improve the resilience of the master to
   malformed messages sent to it by the slave and add support for
   requesting port- forwardings via the multiplex protocol. The new
   stdio-to-local forward mode ("ssh -W host:port ...") is also
   supported. The revised multiplexing protocol is documented in the
   file PROTOCOL.mux in the source distribution.