add ssh_message_channel_request_open_reply_accept_channel to use already existing channel as opposed to ssh_message_channel_request_open_reply_accept.
I do not know a lot about the channels code. Sadly these functions do not have any documentation to clarify that to me.
At least, if they are going to be exposed in the public API, they should get at least basic doxygen documentation. This is strictly not part of this ticket, but I would appreciate to see that in addition to the exposing the function (assuming you know what does it do when you want to use it :) ).