To use knife (slice) aligned to screen, first align the workplane to the screen using ctrl-home, and then use slice.
How to turn it to a hotkey:
tool.set poly.knife on
To use it in Pen mode, use this:
tool.set PenKnife on
Paste this in a text file, save as knifescreen.LXM in your scripts folder (On Windows 7 it something like C:\Users\user\AppData\Roaming\Luxology\Scripts), execute @knifescreen.LXM command in Modo while having your mouse cursor in the perspective window, bind to a key.
It will only fire while having your mouse cursor in the perspective window. After dropping the tool, you can reset the workplane.
I’m aware of Seneca’s knife screen script, but it’s pretty hard to use.