I just found out about the ControlMaster/ControlPath feature of OpenSSH, which allows you to use a single SSH connection to run multiple terminals.
As I often use SSH to use port forwarding to get encrypted and authenticated VNC sessions I instantly recognized that you can't add port forwardings to a remote server to which you already have an established connection. This sucks.
Sometimes later I found out that you can circumvent this limitation by typing ~C in a running SSH terminal session. This opens up a command-line which allows you to add or remove port forwardings.
My quesion now is: How can I add port forwardings on an existing SSH session which is using the ControlMaster/ControlPath feature, without the need to have access to a terminal session inside that SSH session. I need this to enable my script which starts a secure tunneled VNC connection for me to add and later remove its port forwardings.
(I know I could use a terminal multiplexer such as GNU Screen or tmux, actually I'm doing this already. But I like the idea of using just one SSH session for serveral reasons.)