My use case is starting a game stream from my steamdeck after my pc has locked itself. The game will start but the desktop lock screen/screensaver is visible and you can't play or do anything.

You need to SSH in as the active user (from another device like your phone or another laptop) and run:

loginctl list-sessions

You will get a table output with a list of sessions:

SESSION  UID USER  SEAT  TTY  
     38 1000 james       pts/1
     44 1000 james       pts/2
      5 1000 james seat0 tty2

You want to take the one that has a seat allocated and run unlock against it:

loginctl unlock-session 5

Hopefully this will unlock the screen and your steamdeck screen will show the application you're trying to use.

https://askubuntu.com/questions/341014/unlock-login-screen-using-command-line