I created a file /usr/share/X11/xorg.conf.d/90-keyboard.conf containing
Section "InputClass"
Identifier "Defaults"
MatchIsKeyboard "true"
Option "XkbOptions" "ctrl:nocaps,compose:menu,terminate:ctrl_alt_bksp"
EndSection
It looks like it's getting picked up; in /var/log/Xorg.0.log it says
[ 3772.382] (II) XINPUT: Adding extended input device "AT Translated Set 2 keyboard" (type: KEYBOARD, id 10)
[ 3772.382] (**) Option "xkb_rules" "evdev"
[ 3772.382] (**) Option "xkb_model" "pc104"
[ 3772.382] (**) Option "xkb_options" "ctrl:nocaps,compose:menu,terminate:ctrl_alt_bksp"
But it has no apparent effect; caps lock is still caps lock. Am I missing something obvious?