The target image would need to reset that field so that a second reboot puts you back into the bootloader because otherwise you'll be stuck booting that image forever.

The image doesn't need to do it, that's how UEFI bootnext works: the firmware resets the flag before it loads the image.

Well you could change default boot entry in efivars, but if you're relying on firmware for that why not use firmware provided boot menu anyway?

The boot disk isn't guaranteed to be writable.

Even after you’ve already installed a custom boot laser to it? I mean, I agree with you in principle, but we already have the chicken - can’t existence of the egg be assumed?

Consider a DVD that's EFI bootable; we can have whatever bootloader we want on the disc but it is not physically writable

Aside from the DVD issue mentioned in the other person's comment. I have a design for a SED OPAL based encryption setup where the system boots with a read-only boot partition and it only becomes RW as part of the initramfs running (although optionally you can just keep it RO until you need to write to it, but this requires buy-in from the package manager).

I think network booting with EFI would also suffer from a similar problem.