I'm using 2 input sources in GNOME 3.6 and I use a keyboard shortcut Alt + Caps to switch between them. I used the GNOME tweak tool to define this shortcut, and also defined ctrl + space as a shortcut for switching sources via the standard GNOME input sources configuration screen.

Both shortcuts work in regular use, however, non of them work in the lock screen or in overview mode.

Input sources can still be chosen using the mouse in the lock screen or in overview mode. 

The major problem is that if I lock the screen with the wrong input source selected, I have to use the mouse in order to unlock the screen. This means that a computer which doesn't have a mouse, and locked with the wrong input source, cannot be unlocked.

I'm using Sabayon Linux X, which uses GNOME from Gentoo's Portage system.