I'm on Xubuntu 16.04 and there is no way to turn on numlock for lightdm.
I'm used to numlock is always on when I logged in. I have the "numpad:mac" option for setxkbmap to be sure that numlock buttons will print numbers even if it is off but for some reason this option ain't working at login screen. And every time I type my password I stumble on numlock being off. It creates a lot of inconvenience and discontent every time.
First reported on Launchpad at https://bugs.launchpad.net/lightdm-gtk-greeter/+bug/1554734
LP#0: drkwv (bl3-drkwv) wrote on 2016-03-08: