It's the * numpad key, Ctrl-Alt-Shift-8 doesn't work but if my coworker turns on numlock and does Ctrl-Alt-* with the numpad key the screen lock turns off.

It won't work on laptops (most laptops won't have numpad). Ofcourse you can crack into the laptop using an external keyboard.

Most laptops have the Fn button.

Fn+ScrLk is NumLk, and then Ctrl-Alt-8 (on my Lenovo) is exactly the same as Ctrl-Alt-Numpad/8 on a regular keyboard.

Most laptops have a way to use the numpad equivalent by holding down the function key, though I don't have a laptop in front of my to verify that it will still bypass the screen locker with the command.

My Compaq laptop has an embedded numpad, located in the vicinity of the right hand home row. The numpad keys are noted as a key symbol inside a square.

Set numlock will enable the numpad keys by default. If numlock is not set, then fn-numpadkey gets the numpad version of the key pressed. E.g. fn-p is *.

You can do it on regular laptop keyboard too..

Usually the "numpad" is available via "Fn" key - so make sure that Num lock is on and press `Ctrl+Alt+Fn+*`

Multiply is the key on the numpad that only does *.

slock does not appear to be vulnerable.

don't bet on that, it breaks when I try it. maybe you're using a pre-vuln X.

I wonder if it is because I'm using xmonad.

Nope. Doesn't matter what client apps you are running, this would be an X vulnerability.