I honestly don't think Apples intent was for the Hi-Fi to be remarkable via battery use. Due to the weight and size of this unit, its really not a feasible unit for portable use. Sure, you can do it, but I can't see myself lugging this big boy around. I think there are many other solutions that may not provide nearly the sound quality, but will most certainly be lighter and provide much, much better battery useage including rechargeables.
I suppose there are some situations where I could see myself using the Hi-Fi on batteries, but those situations would be far and few between, so buying regular good alkalines would be no big deal. My understanding battery life isn't that good anyway.