openssh client application with multiple channels to a server subsystem

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Here's what I would like to accomplish -

An ssh client application opens a connection to a custom server

The subsystem launchers a server application, but also provides the
server application with apis that allow the application to open
connections (channels) back to the ssh client application (which
itself acts as a specialized server subsystem)

Is something like this possible using apis to openssh, or will it
require that patched versions of the openssh ssh (and sshd)?

A related question is this: does the channel multiplexing support in
openssh/ssh2 prevent one channel's packets from causing the overall
connection to be blocked for tcp window reasons?   In other words, if
there are multiple channels, do they operate independantly without the
possibility of having one channel block the others?

Any help or suggestions would be appreciated.

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