This can be done manually using flashrom [1] tool to read (and write) the SPI (and not only SPI) flash and UEFITool [2] to unpack the corresponding image. I've even done some patching for better support of Intel platforms here [3]. Now doesn't have much time to rebase/cleanup/improve it, but hopefully someone will start from where I've finished. Both tools clearly need more love, and I hope current revelations will help to do that.
I also used flashrom instead because it's very simple to install and use... My problem is trying to verify this against the original (I'm running linux on an old macbookpro), i've got the raw flash dump and extracted the .scap from Apple... but after much searching cannot find a way to extract the raw EFI volume to compare. Any ideas?