That shell command doesn't have much to do with openrc. You only check the PID of /sbin/init, which is almost always 1, even with the default Sys V or systemd or whatever, and then you print a random message, in this case "openrc". Could as well be anything else. Just run the same command as root on a Devuan, Debian, Ubuntu, CentOS or any other Linux that does not even has openrc installed, and check the results for yourself.
Guess what happens on a Ubuntu 18.04 with systemd:

# pidof /sbin/init && echo "openrc" || echo "other"
1
openrc

You might want to try something else, like seeing where init is actually pointing:

# ls -alh /sbin/init 
lrwxrwxrwx 1 root root 20 sep  5 06:59 /sbin/init -> /lib/systemd/systemd

See? It points to systemd on Ubuntu here.

In case it doesn't, you might want to check the executable yourself. For example, on Devuan with Sys V and no OpenRC installed:

# strings /proc/1/exe | grep INIT_VERSION
INIT_VERSION=sysvinit-2.88

See? Here we have Sys V /sbin/init.

Also, if I remember correctly, OpenRC doesn't always replace init...?

copy-root-overlays and postbuild-clean are both defined in helpers... I'm looking at them right now. You should not have received those error messages if you followed all the instructions correctly.

P.S.: I already told you, I build images for raspi 4 with no errors or warning whatsoever.

I don't have a 64-bit Raspberry at this point to test on... and also this:

Yes of course! See the docs.

Run as root/with sudo? modprobe binfmt_misc? Do you see other errors above what you posted?

P.S.: I also clear the dist/ and tmp/ folders before each build. Just in case, because I spotted something weird happening a few times.

FYI I enabled ccache for compiling cache in my fork:
https://git.devuan.org/b3bgd/arm-sdk

Works like a charm, after the first build I get compiling times about 5x better. I needed this because I made round 50 builds from scratch in the last week or so.

P.S.: in that fork you will also find that I put vc tools for raspi 1 & 2 (because they are 32-bit, and there are no 64-bit executables and I don't know if they will work for 64-bit), and I fixed the raspi 2 image & modules.

In your raspberry-pi4.sh, near the top, before function definitions, add the lines:

vars+=(nocompressimage)
nocompressimage="true"

Hi again. I'm gonna answer here directly, maybe others will find this useful too.

If you setup up an amd64 machine to build, everything will be OK, because the script will use cross-compiling to the correct arm architecture (armel/armhf/arm64) depending on your board.

Please look closely at the issues and merge requests on Git. For raspi 2 I found out the partitions were not aligned correctly and the kernel modules were not handled properly, resulting in an unusable image (the image posted on Devuan site right now for Raspi2 doesn't even boot). I fixed the problems, and made a merge request. For raspi 4 things seem in order, but there is a merge request for 4G RAM limit. You might want to look at that.

Also, something else to consider:
1. Even if not mentioned in the readme, you will also need the following packages on the build host installed:

quilt bison flex chrpath gawk texinfo libsdl1.2-dev whiptail diffstat cpio libssl-dev

2. On the arm image itself, after booting, you should create the groups for the bus interfaces, because they are not included:

groupadd --system spi
groupadd --system i2c
groupadd --system gpio

3. See net.ifnames in cmdline.txt and adjust as needed for predictable or classic net names - I myself find the old classic way more predictable on raspi, because it only has one interface of each type.

4. On the raspi 2 image at least, locales are kinda missing. Setting the keyboard doesn't work either.

Hi.
For Raspi2, I just use a virtual Devuan ASCII macine for building, am follow the instructions in the arm-sdk Readme (with some initial quirks, that is).
Where are you stuck?