touch follows symlinks, so this %pre script allows creation of zero-length files (as seen in yum-rhn-plugin-2.0.1-2):

%pre
# 682820 - re-enable yum-rhn-plugin after package upgrade if the system is already registered
export pluginconf='/etc/yum/pluginconf.d/rhnplugin.conf'
if [ $1 -gt 1 ] && [ -f /etc/sysconfig/rhn/systemid ] && [ -f "$pluginconf" ]; then
    if grep -q '^[[:space:]]*enabled[[:space:]]*=[[:space:]]*1[[:space:]]*$' \
       "$pluginconf"; then
        touch /var/tmp/enable-yum-rhn-plugin
    fi
fi

The flag should be stored somewhere else, perhaps in /etc.