Most of these chips can actually be flashed no problem. They just have a fuse/bit that disables read over JTAG and to disable that you have to zero the entire

Most of these chips can actually be flashed no problem. They just have a fuse/bit that disables read over JTAG and to disable that you have to zero the entire device memory. This means that you can put whatever firmware on that you want, but you'll never have access to the old binary code that was running it, so making patches is impossible unless you can either A) rewrite all the firmware from scratch, B) find a way of tricking the existing software into dumping firmware over a port via some sort of overflow, etc or C) glitch the chip into allowing JTAG access with the bit/fuse set (used to be pretty common, more rare these days)