Created attachment 2178 proposed patch Gtk/IMModule xsettings key are supposed to change GTK+'s immodule through air and are used to apply it dynamically. since xsettings specification permits a write access to xsettings manager only and allow one instance for desktop, there are no way to support this key outside XFCE. I'd love to see this support in xfce4-settings, hopefully in 4.6.0 release.
Thanks, committed in rev 29509