Sounds like a good idea until you actually read the article. All you are doing is re-programming the device with different code each time using a avr programmer.
In other words... it's a avr. Any avr can be re-programmed to show up as a HID device so long as someone has written some USB code for it.
What you want is a device that can be re-programmed via the usb cable.. that means easier re-proramming and re-programming on the fly.
There are much cheaper/better options... like a teensy as you've pointed out.
Hell I'm not even a fan of the "arduino" thing. It's just a atmega chip (or similar)... the "arduino" part is the programming language/library and there are tons of languages you can use to program AVRS. Better yet, the code is out there to be messed with... just do a google search.