I have quickly looked at the directory structure and it seems to be OK, so I will have to guess there is something about the way the .iso is constructed, if it is a Microsoft problem.
Indeed, the ISO created by the Windows 8 Upgrade Assistant does not support UEFI boot. The files in the ISO are fine (\efi folder) but the boot images are broken. To boot in UEFI mode from a CD/DVD, there needs to be a special boot image which allows booting
the EFI boot loader in \efi\boot. The ISO created by the Windows 8 Upgrade Assistant only has the standard boot image for normal BIOS mode booting, whereas the ISO of the Windows 8 preview has both boot images, for BIOS mode and UEFI mode.
Then burn the new ISO file to a DVD. I used InfraRecorder and booting from DVD in BIOS and UEFI mode worked fine. Note: I have not tested a whole installation but only until the first installer dialog.