So I have a phone tripod that came with a little remote with a button for taking pictures with your phone

Finally tried it, turns out identifies itself as a Bluetooth keyboard

When you press the button, it sends a "volume up" input, which is already mapped to taking a picture in the camera app

(I can tell because outside of the camera app it just increases the volume)

I heard tell one time of a USB hard drive which automatically installed its driver, by self-identifying as a keyboard when plugged in and then sending inputs as if the user was opening a shell and typing the install script.