Hey guys,

I can compile a lot of programs from source. I can also automatically
package them up into slackware packages.

My question is, though: does anyone know how to remaster the zenwalk
.iso? I kind of want to add a few modifications tailored to my use
case, but I am not sure what to actually do to make these "persistent"
in the .iso. For instance, say I want to add a new package, where
inside the directory structure should I put it and how to register
it, so that it then works when I create a new .iso file? I assume
once I have the directory structure + files, making an .iso from that
is trivial, but I am not sure how to get the installer to also
copy all of these modifications as-is. For instance, to add a default
directory to the then installed /home/ directory of the user (which
would be my generic user "x", I use that directory at /home/, but how
to add it for the .iso?).

There is surprisingly little information available in general in this
regard, but I feel that after many years of using linux distributions
it is time to make my own modifications rather than have to default
to whatever other people decide I should use.

Zenwalk is a distro. You'll have to integrate whatever boot loader they use (Syslinux?), UEFI maybe and it's definitely not for the faint of heart - like me.

A better approach is to go the live usb route imho. You don't need an iso, and there are less hostile ways of getting going that a read-only iso.