Nope, because everything is ultimately dependent on the hardware key in the Secure Enclave and the user's passcode, neither of which Apple has knowledge of. The FBI would be able to create the OS, but they wouldn't be able to load it on the device without erasing it. It's really a brilliant system.
>Nope, because everything is ultimately dependent on the hardware key in the Secure Enclave
The phone in question is an iPhone 5C which does not have a Secure Enclave, only a burned-in hardware ID.
It's also worth pointing out that there's no clear evidence that the Secure Enclave must clear all keys as part of a SE firmware update (just speculation that this would be reasonable).
Compelling production of Apple's private keys would (for better or worse) be a far easier legal question than what this subpoena seeks to compel. Remember the Lavasoft case? The private keys were ordered produced there. The difference here is that what is sought to be compelled here doesn't exist now; it will have to be created through expert engineering and software design work.