Install archiso

pacman -S archiso

Create build tree in /var/tmp

cd /var/tmp; mkdir -p archiso-zfs/{work,isobuild}

Copy in proto ISO

cp -r /usr/share/archiso/configs/releng/. archiso-zfs

Edit archiso-zfs/packages.x86_64 and add:

linux-headers
libunwind
zfs-utils
zfs-dkms
paru

Add custom repo to archiso-zfs/pacman.conf

[archzfs]
SigLevel = TrustAll Optional
Server = file:///var/tmp/archzfs

Create working archzfs directory and populate

mkdir archzfs
cd archzfs
git clone https://aur.archlinux.org/zfs-dkms.git
git clone https://aur.archlinux.org/zfs-utils.git
git clone https://aur.archlinux.org/paru.git

build all packages and make repo

cd paru
makepkg -s
cd ../zfs-dkms
makepkg -s
cd ../zfs-utils
makepkg -s
cd ..
for f in */*zst; do ln $f .; done
repo-add /var/tmp/archzfs/archzfs.db.tar.zst /var/tmp/archzfs/*/*zst

change directory to archiso-zfs and build iso

cd ../archiso-zfs/
sudo mkarchiso -v -w work -o isobuild /var/tmp/archiso-zfs