I have been thinking the same thing, that it'd be nice to add a battery and perhaps an ATU. I suspect others might also have this thought too since these are offered by other radios with similar size and capabilities.
My preference would be that the QRP-Labs supplied case not allow extra room for these however. I would prefer that these and the case to hold them be left as a project for the user. Why? Because I think that the QRP-Labs case should minimize (within reason) the size. Sure there might be room for some smaller add-ons but even a small battery capable of multiple watts and a small tuner will take up a fair bit of space. I don't think the users, who don't want those, should have to put up with the extra size. This is a nice *little* radio. Let's leave it such.
I see Hans has posted dimensions and I don't think that they leave a lot of room. I guess it can be a challenge for those who want to take it on : - )
This does lead to a request however. I'm hoping that there will be an option to buy just the electronics (all-band radio incl. PA and relay/filter board) without the case. I haven't bothered to mention it since its hardly a product design decision and, although Hans has only mentioned a couple of price points (single band and all band with case), I would assume he's also open to other reasonable options. But since you mention this I'll make the request now.