Okay, got curious and played with it... it seems like just this will do it?

    /data/data/com.termux/files/usr/etc/proot-distro/alpine-pmos.sh


    DISTRO_NAME="Alpine Linux with postmarketos added"
    DISTRO_COMMENT="Rolling release branch (edge)."

    TARBALL_URL['aarch64']="https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/aarch64/alpine-minirootfs-3.20.2-aarch64.tar.gz"
    TARBALL_SHA256['aarch64']="6ae4d30a75dacd5991a1c1ab13453a119b54a437d236b1680b6d844f015d141b"

    # based on https://github.com/termux/proot-distro/blob/master/distro-build/alpine.sh
    distro_setup() {
        mv ./etc/apk/repositories ./etc/apk/repositories.initial
        echo "http://dl-cdn.alpinelinux.org/alpine/edge/main" > ./etc/apk/repositories
        echo "http://dl-cdn.alpinelinux.org/alpine/edge/community" >> ./etc/apk/repositories
        echo "https://mirror.postmarketos.org/postmarketos/master" >> ./etc/apk/repositories
        run_proot_cmd wget -O /etc/apk/keys/build.postmarketos.org.rsa.pub https://gitlab.com/postmarketOS/pmbootstrap/raw/master/pmb/data/keys/build.postmarketos.org.rsa.pub
        run_proot_cmd apk upgrade
        run_proot_cmd apk add shadow-login
        #run_proot_cmd ln -sf /var/cache/apk /etc/apk/cache  # ?
    }

If you're talking about guides and cleanup and such, I'm concerned that I've missed something - is it not just that easy? If I was publishing a wiki page or something, literally the only things I'd include would be the initial step to install proot-distro in the first place, and fleshing out the rest of the tarball urls so it works on more than aarch64.