The tl;dr purpose of my e-mail was 'Is implementing UEFI Secure Boot for the EFI playloads
I think that you'd better explain why you stick to *UEFI* secure boot.
The main reason is distro support. Since distros use a number of different ways of booting up on arm boards, using UEFI is the obvious way to unify that (and alrady supported on some) regardless of the bootloader. UEFI secure boot provides a common approach to security instead of 'per bootloader' solutions
Thanks /Ilias