I necro'd a 2 year old thread, since the information was outdated and therefore didn't answer my question (how to run a command at boot).

Following the new instructions, I edited my /etc/66/rc.local and added the shebang and the line I wanted to run; I chmodded it; and then opened /etc/66/boot.conf and changed the rc.local and swap lines to "yes". Finally, I rebooted and was greeted with [Starts stage1 process...] without continuance. The kernel loaded, since REISUB still works, but I can't get any further than the beginning of stage 1.

I tried chrooting into my drive from the installer, but the root partition was different and changes didn't remain, though my home partition was as usual.

What should I do?

EDIT: the answer was not to chroot, but rather change the file directly as the installer root:

"# " indicates root 
# mount /dev/sda7 /mnt
# nano /mnt/etc/66/boot.conf
Is your swap partition specified in /etc/fstab?
If it is not the boot process tries to turn swapon on a file/partition that is not defined, and this would be an error.
In general to trouble shoot a booting problem if you don't get a direct error message that reveals the problem do this:

1 edit /etc/66/init.conf and switch verbosity to 4
2 reboot
3 go to tty12 (hopefully it works) and login as user (not root)
4 cat /run/66/log/0/current

I have a small shell with these two lines that scans the long log file and gives me the juicy parts. Usually the first error (if many) is the one causing the trouble, but in any event you try to fix one thing at a time, as sometimes later errors are caused by the first.
cat /run/66/log/0/current | grep unable
cat /run/66/log/0/current | grep fatal

Also by changing one thing at every retry helps you identify what was wrong and how it was fixed. If you change 3 different things at once you don't know which fixed the problem.
fungal_net wroteIs your swap partition specified in /etc/fstab?
Yes, it is. Simply flicking back LOCAL=!yes to LOCAL=!no let me boot without issue. I don't believe there was anything particularly wrong with my shell script, but perhaps I'm mistaken.
For future sake, here's what I added to my /etc/66/rc.local excl. comments 
# !/bin/sh
cat /run/66/log/0/current > /etc/bootlogs/"$(date)".txt
fungal_net wroteIn general to trouble shoot a booting problem if you don't get a direct error message that reveals the problem do this:

1 edit /etc/66/init.conf and switch verbosity to 4
2 reboot
3 go to tty12 (hopefully it works) and login as user (not root)
4 cat /run/66/log/0/current
Decided to revert my fix and try to identify the issue, using the quoted method. The entirety of /run/66/log/0/current can be viewed here, if anyone is interested. But here are just the "fatal" and "unable" parts of the output.
Unable: 
$ 2020-09-24 23:30:30.355512024  foreground: warning: unable to spawn /etc/66/rc.local: Exec format error
$ 2020-09-24 23:30:30.356984433  s6-rc: warning: unable to start service local-rc: command exited 1
$ 2020-09-24 23:30:30.357160845  66-dbctl(src/lib66/ssexec_dbctl.c: ssexec_dbctl: 343): fatal: unable to start services selection
Fatal: 
$ 2020-09-24 23:30:30.357160845  66-dbctl(src/lib66/ssexec_dbctl.c: ssexec_dbctl: 343): fatal: unable to start services selection
$ 2020-09-24 23:30:30.358141274  rc.init: fatal: please see the log at /run/66/log/0/current
unrelated and I have been criticized about my dirty solution for resolv.conf, I lock it with chattr +i /etc/resolv.conf

chattr -i /etc/resolv.conf must be issued before it can be rewritten or deleted.
lsattr shows whether it is locked or not.

Then dhcpcd throws errors in the log file being unable to rewrite it to the router's default.
If you search resolv.conf on the forum you will see a couple of suggestions by Eric on how to do this right with a resolv daemon.

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