I've succeeded in mapping ctrl+alt+z to a custom binding (gnome-system-monitor).
When I change it to ctrl+esc, or ctrl+alt+esc, it does nothing. I looked through the other bindings mapped in keyboard preferences,and don't see any conflicting bindings.
Does anyone have any ideas about why this wouldn't work?
Edit:
Thanks to @Redbob's suggestion, I found the existing binding for ctrl+alt+esc in the dconf-editor. No such luck with the plain ctrl+esc binding: I also failed to find the ctrl+esc binding in the dconf settings on terminal (gsettings list-recursively | grep "Escape" as suggested by this question).
The conflict for ctrl+alt+esc came from this dconf entry:
org.gnome.desktop.wm.keybindings cycle-panels ['<Control><Alt>Escape']
