jpilot: Actually it has been
invented already.
Those boxes look kinda big? Like placing a small-ish desktop PC case under your laptop. Not very attractive at least to me.
If it was an enclosure not much bigger than the actual desktop graphics card, maybe then I might be interested. Something that fits neatly into your laptop bag, along with your laptop. And yeah, it should be standardized so that it works with different laptop vendors.
As far as gaming laptops go, I don't care that much how usable they are with battery power. I always use them with mains power anyway. The battery is mostly nice in case there is a power cut, so that I have enough power to save any stuff and shut the system down properly (or put to sleep mode, or hibernate, or whatever). E.g. in Thailand that is quite common, trying to use a desktop PC there without an UPS is a nightmare.