If you want to do this the traditional way, you would setup a user, then properly secure it (you might end up becoming a proxy) and then set a ForceCommand. After that, verify your config works, then set the password for that user to an empty password. After that, allow empty passwords on your ssh config so you can ssh to this user without any authentication.

This is how eg anoncvs works. After that point, it's just a matter of writing the application you want to run, with stdio being connected to your user.