I can't see why it won't work, as long as you will get a min of 8x lanes in that x16 slot, you shouldn't have any problems. Both my HD7970M cards are running PCIe 3.0 x8 x8. Why don't you get a HD7970M as these are faster with current drivers, and a lot cheaper than the GTX 680M.
There was a guide posted, amounts to adding a line to an INF so that the generic Nvidia driver install recognises it as a valid card. The fact that this is needed does somewhat surprise me though, a 680M is as any other surely. Err... I'm based in the midlands... of the UK. I like a road trip as much as the next guy, but that seems excessive! I got the impression from reading around that the 680M would be a much better option for 2560x1440. Not that that's entirely relevant, mind. A chap on the HP forums tried out a 7970M first, and couldn't get the cooler to fit properly apparently. This surprises me, as I would have though an MXM3.0B card would be an MXM3.0B card, but supposedly not. I figure sticking to Nvidia would be the best way to minimising the risk of incompatibility anyway.