I wonder if the Firmware supports such upgrades. I`m not a pro with notebooks, but did already someone try to upgrade his/her nb?
I wouldn`t even think about spending 425$ to buy a graphic adapter that might or might not work.
Even if the board fits in (which is rather likely) and there is no special requirement on the firmware side of the story, waste heat will be a rather big problem. For instance, a 3438M with two harddiscs (wich ist the most common config) has already heat problems-using a graphic adapter that is producing more waste heat than the one it was build for will be problematic for certain. Best case, the fan runs most of the time, worst case you would have to let it cool down every few minutes.
So if anyone is going to sell these MXM III boards, it would only be reasonable to demand a guarantee that an upgrade on certain NBs will work.