I do not believe it is doable, not at the D800 price point, and not without a new sensor. And Canon is not losing sleep over losing customers to Nikon over the D800.
I think its highly speculative to think Canon is losing enough customers to matter to the D800. Keep in mind, despite its great sensor, the D800 (and D4, for that matter) have had quite a few problems of their own, from a very yellow-green LCD, to RAW images that mirrored that yellow-green hue, CF card compatibility problems, tethered shooting issues, a couple AF issues (that has caused many a D800 owner quite a lot of problems requiring multiple returns to Nikon support), and on top of all of that, Nikon throughout it all has often denied, even in the face of mass evidence by the online community, that the problems were even real...and demonstrated they have vastly inferior support to Canon, making their customers jump through a myriad of hoops and hoopla just to get some necessary service.
Sorry...I don't think Canon has anything to lose sleep over. The only edge the D800 has is ISO 100/200 DR and some extra megapixels, and it seems the extra megapixels deal won't be much of an edge for long. DR is important, but I would take Canon's "crappy" sensor tech and their stellar customer support over Nikon any day.