OK, maybe I'm dumb, but wouldn't it be easier to just use a powered USB hub (12 volt of course).
Because you will loose the power to the USB devices as soon as you take the key out. Shutdown controller do have some delay before shutting down as soon as the ignirion is gone.
What if the PC is trying to write to the USB HDD before shutting down? but you already took the power away from it?
I guess, yes it can work for some setup