People have thought of such things before, however it is such an incredibly small niche market. Most people wouldn't bother because they're normally already holding the device, or at very least within arms reach.
As far as getting it on existing devices, it isn't as simple as you think. If you're to use USB, then the device has to have USB host capabilities. Most ebook readers currently available do not have this. If that is the case, then you'd have to have software to talk to the device, and respond to it, even if it adheres to USB HID standards. That part is easier to do, but still a bit of a challenge.
Basically though, for most companies, too few people would be willing to buy such an accessory to make it worth while to develop.