SecureBoot might be more useful than UEFI on SBC like Pi.

The grub EFI shim is signed, but does or doesn't verify kernel image and initrd and module (and IDK optionally drive and CPU and RAM hw) signatures?

mokutil does module signature key enrollment. Kernel modules must be signed with a key enrolled in the BIOS otherwise they won't be loaded.

To implement SecureBoot without UEFI would be to develop an alternate bootloader verification system.

But what does grub or uboot or p-boot do after the signed grub shim is verified?

mokutil and these commands don't work without UEFI:

  mokutil --sb-state
  mokutil --help
  mokutil --import key.der
  mokutil --list-new
  reboot

  efibootmgr
  efivar

  fwupd
  fwupdtool
  fwupdmgr get-updates && \
  fwupdmgr update

  tree /sys/firmware/efi

  systemctl reboot --firmware-setup