Hi Darius,
Yep, I think our understanding ties up!
For your first paragraph, section 12.3.2.1, "ISO-9660 and El Torito", the full text that made me double-take was:
To boot from a CD-ROM or DVD-ROM in the boot services environment, an EFI
System partition is stored in a “no emulation” mode as defined by the “El Torito” specification. A
Platform ID of 0xEF indicates an EFI System Partition. The Platform ID is in either the Section
Header Entry or the Validation Entry of the Booting Catalog as defined by the “El Torito”
specification. EFI differs from “El Torito” “no emulation” mode in that it does not load the “no
emulation” image into memory and jump to it...
I can't quite decide if that means "An EFI System Partition stored on CD/DVD media must be stored in no emulation mode or it won't be booted" or not. As you say, according to 3.4.1.1, anything that is recognised as a valid FAT32 Image (i.e. supporting Simple File Protocol) should be tried, however, it does refer to section 12 as defining the format of the file system - I wonder if the spec authors intended for emulation type to be part of these requirements? If so, they didn't bold and underline it very well.
Since the emulation type is really irrelevant if the section entry is EFI (it's either a valid file system, or it isn't), booting the disc regardless of the emulation flag does seem like the sensible thing to do.
The hardware in question is a dual EFI/BIOS setup, which will boot MBR based disks too, sometimes even when you don't ask it to. It could be as simple as the firmware authors decided that since floppy emulation means nothing under EFI, that it must be MBR-based, so tries that (disregarding the EFI section entry), doesn't find an MBR signature and moves on to the next device.
Anyway, I'd be interested to know what you think, and wanted to bring it up in case it was an issue, albeit a very minor one.
Thanks,
Antony