PeterH:
You'd need to use a USB driver rather than just wire the pins up as you suggest, but people have programmed Arduinos to emulate a keyboard and that seems to be what you're asking for here. (Personally I would think that hitting a few keys on a physical keyboard would be easier than plugging in a USB stick, and I wonder how many times you can plug things into a USB port before you wear the contacts out, but those are separate issues.)
I think Nick Gammon mentioned he has a Leonardo plugged to his computer in addition to the regular keyboard, and he has 10 or so buttons, that when he presses a button, it enters in a standard amount of text for one of the newbie questions he frequently answers, such as how to get the lowest power Arduino and make the batteries last longer.