My uninformed understanding, is that you allow people to login into an actual user account within the system, but then you replace their shell (usually you change something like /bin/sh to /usr/bin/ssh_application in the entry for that account within the /etc/passwd file), so when someone logs into that account, instead of running an "open" shell, they get the application instead, and when the application exits, the session ends
So essentially you should be able to create an ssh application with just a shell script
So essentially you should be able to create an ssh application with just a shell script