The way to do this would be to write a client side javascript function in one the onKeyDown/onKeyUp/onKeyPress events of the field ... in that function you can do something like this:
if(window.event) { // IE
keynum = e.keyCode
} else if(e.which) { // Netscape/Firefox/Opera
keynum = e.which
}
keychar = String.fromCharCode(keynum)
then keychar will be the character they entered. I believe in onKeyDown, you can return false, and it will not record the character, but you'll have to experiment to make sure.