I know that in Fedora, if using openbox, there was an input method section in /etc/xdg.
You have been right, there it is! In a bunch of other "mandatory" programs.
]]># pacman -Q | grep ibus
ibus 1.4.2-1
ibus-anthy 1.2.7-1
libusb-compat 0.1.4-2
libusbx 1.0.14-1