It wouldn't work, or otherwise the DOJ would have set up an automated cloning of the phone and trying 10 password attempts at a time. What I've gathered from reading the various articles is that each IPhone has it's decryption key stored in hardware, with no way to copy it. Easily, that is, I suppose they could break open the chip and try and chart the pathways using a scanning tunneling microscope, but that would take a great deal of effort, and it would be easy to accidentally destroy the key doing so.
So what they're trying to do is get Apple to create a signed version of IOS which when installed, won't wipe the phone after 10 failed password attempts. They obviously have a way to install new versions of IOS even without a user entering a password. Which is actually sensible, if you think about it.. when an IPhone accidentally gets bricked when an update fails (or deliberately, witness the recent "Error 53"), it makes sense for there to be a way to force reload a fresh version from scratch without destroying the user's phone. The only reaason the DOJ needs Apple's help is, likely, that there's protection in place on IPhones to prevent unsigned code from being installed. Hence they want Apple to create the "poisoned" version of IOS that, when installed, makes doing a brute-force cracking of the phone easier.