But on essentially all existing UEFI systems you can trivially overwrite the "db" keystore in flash and install anything you please.
Also most (all?) UEFI systems are not locked to Windows and allow customizing the keystore via the firmware console interface anyhow.
All of them.
The Secured Core machines still allows you to reset Secure Boot into user mode as mandated by the spec.
I don't own ARM machines with UEFI so I have no clue.
But on essentially all existing UEFI systems you can trivially overwrite the "db" keystore in flash and install anything you please.
Also most (all?) UEFI systems are not locked to Windows and allow customizing the keystore via the firmware console interface anyhow.