Some good news:
There is a method of booting with any unsigned
EFI file (for example Linux GRUB) on Asus VivoTab devices with the recent firmware.
This also allows loading a "cracked" bootmgfw.efi that does not check for signatures of Windows kernel modules, and after patching the ci.dll - you'll be able to run any app or load any unsigned driver (even the boot-mode driver, unlike the 8.0 jailbreak).
The limitations of my method:
- It works only on Asus VivoTab RT tablets. Surface is not supported due to differences in UEFI firmware modules.
- Bitlocker should be disabled (manage-bde.exe -protectors -disable c: )
- There would be a line stating that secureboot is incorrectly set up, you can see it in the lower-right corner of the screenshot.
- The most inconvenient thing: it requires a FAT32-formatted USB stick with a "hack" file to be inserted on boot.
And, obviously, the "hole" could be closed by Asus in one of the next firmware updates. So Windows Update should be switched to manual mode (8.1 allows to select this from GUI).
So this should be considered as a temporary method until something universal would be found. But it can be used to start developing Linux (or android) for Tegra3.
I'll publish the instructions after 8.1 would be released.