I'm not sure how to get an external keyboard to work but the with an internal keyboard like the Hacker's Keyboard if you long press the characters (a,e, i, o, u) you will get the special keys you want and Android does recognize them. Which is why it seems strange that an external keyboard wouldn't produce the same results. This was even brought up in the Edge Forum two years ago:
http://somestupidwebsite.com/downloa...t&goto=newpost