Linux: Connect to another a terminal session

I can use screen or tmux to connect to other terminal sessions, or to share them. But this work only when we first know we will use them.
As root can access everything, is there a way to do the same thing without running screen ? Some pipes through tee or so.

For example to remotely take over a console or ssh session someone left open. Allowing to see what's on and close things correctly.

I only know a systemtap recipe. Hawfully basic, but maybe someone skilled could improve it.

Else you can have a look at conspy. I know you use Debian, just apt-get it. It only has access to the console, not ssh.
ttysnoop is nice but, as you stated, needs to be installed first.

In the past I used TTYsnoop, which still seems to be included in Debian and Ubuntu.