How to connect to an already open Terminal session on another Mac
Screen is the best way to do it. The command is available under both Mac OS X 10.7 and 10.8.
I configured the console sessions in GNS3 to use Terminal and screen with the following command:
/usr/bin/osascript -e 'tell application "terminal" to do script with command "screen telnet %h %p ; exit"'
This can be found in GNS3>Preferences>General>Terminal Settings
Then I list the screen sessions in my remote ssh session with the command:
screen -list
and... finally connect to the required router with:
screen -D -r <name of session obtained with "screen -list" above>