From a look at the SD card on Wikepaedia - the font of all knowledge - there's part of the SD card that has some sort of DRM protection. Maybe that could be used to protect a special SD card so the magic sauce can't be copied.
Many years ago I was asked to put some protection in our system. I put a code in the memory of the RTC (the only writeable but permanent memory of the main board), in the boot sector of the HDD and in a file on the HDD. As long as two out of three were the same you were OK.
Our service and sales people hated it and eventually the protection was disabled. it was trivial to unlock, replace the call to the check function with a NOP.
AFAIK nobody reverse engineered it, the word leaked out that you could fix it with a hex editor but that was it.
It wasn't an iPhone of course