This isn't really a new concept, but previously I've seen this attack used from USB memory sticks which modified firmware. The idea being that you could use them as sort of dead drop and the target would still be able to see that it's fully functional storage device and it would still act like HID (e.g. keyboard) and execute the commands.

But since Teensy is a different beast, maybe there could be some new neat things you could do with it.