WELCOME!
That's definitely possible and easy to do. The teensy should be able to mimick any keystroke sequence, i'm pretty sure. In fact it can be used for super malicious things since it's automatically recognized as an HID device and doesn't have any drivers to install. There's a fun DEFCON video about it =)
It has also been used as something more than a POC.
A pentester team break into a system, by sending staffers of the system "free" commercial usb sticks, or usb mouse (or did they drop them on the floor, I don't remember). One of the staffer couldn't resist and plugged it in the office computer. The device worked as mentionned, but after a little time of inactivity, it acted like a keyboard, and send the right keystroke to code/implement a backdoor.
Such a method is completely unavoidable, since it simulates user input on a hardware basis, outside of the PC. At least as long as we don't have signed periphericals (you can't plug a new usb device in before the administrator whitelists it for example). Or smart employees (as if this is possible).
