I wonder how hard it would be to create various firmware updaters for drives or other devices.

I'd be cautious here because you very easily could brick the hardware. For writing a PC driver it's a lot safer since you can just fail repeatedly until it works.

Firmware is a lot harder for a whole host of reasons but it is possible / I've done it.

The odds of success go up a LOT when the stock firmware is easy to extract and reverse engineer.