you can already hit backspace to reply... which is weird, I know. I guess that's because R is auto walk/ahead full. I don't know all of the circumstances to make it work but there is also an autofill on /tell, like to your friends on fleet people and what not so you can /tell bo and Bob@mynamesucks will auto fill in those instances. all you have to do is select it.
Though for the life of me... why doesn't the chat channels refresh automatically when you change toons? Switching from one toon to another causes /fleet to vanish from the list of channels.