HEUREKA, it lives!

The ramdisk is now mounted using another SimpleFileSystem. However, this comes with an external dependency on an ISO9660 driver, which must be loaded before bootiso.efi is launched. Maybe I can automate that too.
But yes, the ISO correctly loads its bootloader, and I get to the GRUB menu. Sadly, once Linux is loaded, there are errors about a missing fd0 device, which is probably one of the harder parts of this project (if it wasn’t hard enough up until now).