The good news is that, yes, you can define a compose key in Xubuntu. The bad news is that you have to do a bit of minor surgery on a configuration file.
The configuration I wrote about earlier, it seems, is only good for Ubuntu and its Gnome desktop. On Xubuntu, which uses Xfce, you have to edit the
/etc/X11/xorg.conf
config file.Look for this code snippet:
Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "CoreKeyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "us"
Option "XkbOptions" "lv3:ralt_switch"
EndSection
and replace
Option "XkbOptions" "lv3:ralt_switch"
with
Option "XkbOptions" "lv3:ralt_switch,compose:rwin"
This should assign the right Windows key to be the compose key. Operation is as documented previously.