When attempting to swap control and capslock keys using process described in http://wiki.linuxquestions.org/wiki/Swap_Caps_Lock_and_Ctrl (ctrl:swapcaps or ctrl:nocaps) XFCE overrides that option. The option is effective when running plain X, showing that the option was set properly. Getting /usr/lib/xfce4/mcs-plugins/keyboard_plugin.so out of the way resolves the problem, so that the XkbOptions take effect.