It works! Immediate poweroff. FDE password required on start.
Host: Debian 8 + XFCE4
Works regardless of which window has focus. Tested with focus on Host Terminal and also with focus on a Virtualbox Whonix Workstation VM window.
No effect: Qubes 3.1 (Fedora 20) + KDE4
Does not work in dom0 window or in a Debian VM.