{
  boot.postBootCommands = ''
    # On the first boot, resize the disk
    if [ -f /disko-first-boot ]; then
      set -euo pipefail
      set -x
      # Figure out device names for the boot device and root filesystem.
      rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
      bootDevice=$(lsblk -npo PKNAME $rootPart)
      partNum=$(lsblk -npo MAJ:MIN $rootPart | ${pkgs.gawk}/bin/awk -F: '{print $2}')

      # Resize the root partition and the filesystem to fit the disk
      echo ",+," | sfdisk -N$partNum --no-reread $bootDevice
      ${pkgs.parted}/bin/partprobe
      ${pkgs.bcachefs-tools}/bin/bcachefs device resize $rootPart

      # Prevents this from running on later boots.
      rm -f /disko-first-boot
    fi
  '';
  disko = {
    extraPostVM = ''
      ${pkgs.zstd}/bin/zstd --compress $out/*raw
      rm $out/*raw
    '';
    devices = {
      disk = {
        disk1 = {
          imageSize = "10G";
          type = "disk";
          device = "/dev/mmcblk0";
          postCreateHook = ''
            lsblk
            sgdisk -A 1:set:2 /dev/vda
          '';
          content = {
            type = "gpt";
            partitions = {
              firmware = {
                size = "30M";
                type = "EF00";
                content = {
                  type = "filesystem";
                  format = "vfat";
                  mountpoint = "/firmware";
                  postMountHook = toString (pkgs.writeScript "postMountHook.sh" ''
                    (cd ${pkgs.raspberrypifw}/share/raspberrypi/boot && cp bootcode.bin fixup*.dat start*.elf *.dtb /mnt/firmware/)
                    cp ${pkgs.ubootRaspberryPi4_64bit}/u-boot.bin /mnt/firmware/u-boot-rpi4.bin
                    cp ${configTxt} /mnt/firmware/config.txt
                  '');
                };
              };
              boot = {
                size = "100M";
                content = {
                  type = "filesystem";
                  format = "ext4";
                  mountpoint = "/boot";
                };
              };
              root = {
                name = "root";
                size = "100%";
                content = {
                  type = "filesystem";
                  format = "bcachefs";
                  mountpoint = "/";
                  postMountHook = toString (pkgs.writeScript "postMountHook.sh" ''
                    touch /mnt/disko-first-boot
                  '');
                };
              };
            };
          };
        };
      };
    };
  };
}

Is this perhaps another way that disko's architecture fails because of the hostPlatform issue?

corpo-disko-images> qemu-system-aarch64: could not load kernel '/nix/store/fqwx7jpif67dzqhiggnnj69nhch8jp23-kernel-modules/vmlinuz.efi'
error: build of '/nix/store/zzvgpqhc34ha13p2a70l2s5ya324safa-corpo-disko-images.drv' on 'ssh-ng://matthewcroughan@aarch64.nixos.community' failed: builder for '/nix/store/zzvgpqhc34ha13p2a70l2s5ya324safa-corpo-disko-images.drv' failed with exit code 1;
       last 2 log lines:
       > unable to handle EFI zboot image with "zstd22" compression
       > qemu-system-aarch64: could not load kernel '/nix/store/fqwx7jpif67dzqhiggnnj69nhch8jp23-kernel-modules/vmlinuz.efi'
       For full logs, run 'nix log /nix/store/zzvgpqhc34ha13p2a70l2s5ya324safa-corpo-disko-images.drv'.

corpo-disko-images> setting up /etc...
corpo-disko-images> Initializing machine ID from random generator.
corpo-disko-images> File system "/boot" is not a FAT EFI System Partition (ESP) file system.
corpo-disko-images> Traceback (most recent call last):
corpo-disko-images>   File "/nix/store/hk9khalas7ig9lpd8xygb00jgx9kjv2d-4d4f3f52hgwgk63d4fjg1rj4c1jdg9mn-systemd-boot", line 394, in <module>
corpo-disko-images>     main()
corpo-disko-images>   File "/nix/store/hk9khalas7ig9lpd8xygb00jgx9kjv2d-4d4f3f52hgwgk63d4fjg1rj4c1jdg9mn-systemd-boot", line 377, in main
corpo-disko-images>     install_bootloader(args)
corpo-disko-images>   File "/nix/store/hk9khalas7ig9lpd8xygb00jgx9kjv2d-4d4f3f52hgwgk63d4fjg1rj4c1jdg9mn-systemd-boot", line 292, in install_bootloader
corpo-disko-images>     subprocess.check_call([f"{SYSTEMD}/bin/bootctl", f"--esp-path={EFI_SYS_MOUNT_POINT}"] + bootctl_flags + ["install"])
corpo-disko-images>   File "/nix/store/vbfsfi87f7ks3hndbvn72n4334f9xll0-python3-3.11.9/lib/python3.11/subprocess.py", line 413, in check_call
corpo-disko-images>     raise CalledProcessError(retcode, cmd)
corpo-disko-images> subprocess.CalledProcessError: Command '['/nix/store/wh5wyrs4nxdx7kh3ip0psn11rc2kknl1-systemd-255.4/bin/bootctl', '--esp-path=/boot', '--no-variables', 'install']' returned non-zero exit status 1.

corpo-disko-images> + /nix/store/gc39rhvazg28s7y80c4c0a24x3hsv92p-postMountHook.sh
corpo-disko-images> + rm -rf /tmp/tmp.qOc2Qrmj2x
corpo-disko-images> warning: the group 'nixbld' specified in 'build-users-group' does not exist
corpo-disko-images> cp: cannot access '/nix/store/flqdgxi79hgv69k8vc1rdcxvn30m0i3r-perl5.38.2-HTTP-Cookies-6.10': Cannot allocate memory
corpo-disko-images> cp: cannot access '/nix/store/fzh5aadrdl76f7cn930ls979x6cgblyd-libsoup-2.74.3/share/locale/gl/LC_MESSAGES': Cannot allocate memory