This was exactly my thought. You would build a small circuit board with contacts on the bottom corresponding to the footprint of the NAND flash chip. On top of

This was exactly my thought. You would build a small circuit board with contacts on the bottom corresponding to the footprint of the NAND flash chip. On top of the board would be either a flash chip that is denser than the original flash, along with some circuitry that allows you to electrically disconnect it from the phone and reprogram it externally, or a high speed serial interface with a cable connecting the board to an external device—such as an FPGA connected to enough DRAM to have an image of the entire flash—that lets you emulate the NAND flash.