The https://linuxhint.com/use-etc-rc-local-boot/ web page refers partly to systemd and partly to sysvinit.
BionicPup64 uses busybox init.
EasyOS probably uses busybox init.
(but it might not, I have a somewhat old version of EasyOS on a usb flash drive, so I'm not the person to ask about EasyOS details.)
busybox init.is similar to sysVinit.
Puppy doesn't really have run levels
but busybox knows when XWindows (wallpaper and buttons to click, etc.) is running.
So ctrl+alt+del does nothing when X is running.
If you kill X from the menu or by pressing ctrl+alt+backspace,
then ctrl+alt+del will reboot Puppy.
If the bootloader is running, you can poweroff your computer by pressing the computers power button.
(a normal press, no need to hold the power button for 5 or 10 seconds.)
Busybox init will eventually execute the shell script /etc/rc.d/rc.local.
linuxhint.com refers to /etc/rc.local
Puppy does not have this file, Puppy (and EasyOS) have /etc/rc.d/rc.local
You can put stuff in /etc/rc.d/rc.local (not X applications, X isn't running yet.)
If the firewall rules are configured in /etc/rc.d/rc.local.
then you could put those iptables commands to DROP packets from a specific url address at the bottom of /etc/rc.d/rc.local
If the firewall rules are set after /etc/rc.d/rc.local executes,
(for example, in /etc/init.d/)
Then putting an iptables command to block an ip address in /etc/rc.d/rc.local won't work, because setting the firewall rules will erase the firewall chains (including your iptables rule blocking that ip address)
So you would need to find somewhere else, like /root/Startup.
And you might put a sleep instruction in a script, to be sure it executes after the firewall rules are set up.
if there is a gui (buttons to click) jn the menu, you might prefer to use that,
If you prefer CLI, You just need to edit /etc/resolv.conf.head once.
You would need to use iptables to block an ip address every time Puppy (or EasyOS) boots.
Does this work with iptables and network configurations?
The iptables to block an ip address and configuring the dns nameservers, bypasses the configuration that Puppy (and Easy) set up.
If you want to remaster EasyOS, I really don't know.
It is possible, of course.
If you want to run Easy with no access to the drives, but have all your configurations set up (saved)
I don't know how to do that.
Easy must have had access to easy.sfs on a hard drive or flash drive, to be able to copy it to ram then mount the file system in easy.sfs.
Or the OS might have mounted the easy.sfs file that is on a hard drive or flash drive.
So you want persistence, and run in ram with the hard drives and flash drives locked so they can't be mounted.
AFAIK, that can be done. But I don't know exactly all the details.