When there's a will there's a way. Personally I would go for unpacking the 64bit userland+packages in a subdirectory, boot once into a rescue userland which ju

When there's a will there's a way. Personally I would go for unpacking the 64bit userland+packages in a subdirectory, boot once into a rescue userland which just switches the directories around, then boot normally. GRUB already has support for "boot once in this alternate configuration", so it's all there.