thanks for your responses.
i've just had success by adding a few lines to /etc/X11/xinit/Xclients, in the section at the beginning where it's determing the preferred desktop, so it now looks like:
if [ -f /etc/sysconfig/desktop ]; then
if [ "$DESKTOP" = "GNOME" ]; then
elif [ "$DESKTOP" = "KDE" ]; then
elif [ "$DESKTOP" = "ion3" ]; then
...and then also changing /etc/sysconfig/desktop to:
yes, ion3 is a window manager, not a desktop manager.... i don't use a desktop manager, but that seems to be immaterial to this method.