You could add the if...fi line to /etc/rc.d/rc.local.
Slackware's boot scripts can be found in /etc/rc.d/, and you can use them to control your services, for example:
/etc/rc.d/rc.ssh start # start ssh (also done at boot time)
/etc/rc.d/rc.ssh stop # stop the ssh service
The slackware scripts use a if [ -x ... ]
line to run the script only if it has executable rights. In other words, you can disable serviced with the chmod command:
chmod +x ...
chmod -x /etc/rc.d/rc.inetd
restores the executable permissions, thus allowing you the run the script again. (and it will run at a next reboot too)