That's not entirely true. The syslinux class does indeed a lot of this, I was able to experiment with this using the live hddimage. However, this mechanism ends up creating 2 different partitions, one containing the bootloader, and one containing the rootfs. I would like to have them in the same partition. I was able to find how to do it by scripting, but it's outside of the build environment. My script looks like this:
loop_dev=$(losetup --find --partscan --show "my-image-genericx86-64.wic")
if [[ -z "$loop_dev" ]]; then
    echo "Failed to set up loop device!"
    exit 1
fi

if [[ "$arch" == "amd64" ]]; then
    echo "Setting up Extlinux"
    mount_dir=$(mktemp --directory)
    mount ${loop_dev}p1 ${mount_dir}
    extlinux --install ${mount_dir}/boot/syslinux
    umount ${loop_dev}p1
fi