I rarely make comments here in Nikonians. You may have already put your project to bed, but hope this helps you for future reference.
I am a graphic designer and photographer by profession and routinely shoot for and design displays from 6' indoor banner-ups to billboards. I have used both the D700 and D800 and quite honestly either is fine, but (and note I am posting this in the D700 forum)... I prefer the D700 hands down...
This won't affect you but if you have an experienced designer doing the piece, he/she will know it has everything to do with file handling. There isn't a huge quality difference between 12+ and 36+ MP in the end, simply because of the process used to output the product. A billboard file size will run into X-Gbs at only 65-85 ppi for example (unless it is vector), so why add to the storage load when you don't have to? Your 6 X 12 side panel won't be output at full rez anyway. Most vendor outputs run that size at 200 ppi max (usually 150 is more common). Larger displays are meant are to be viewed from a bit of distance. I used to use Fractals to upsize and now use OnOne's version called Perfect Resize.
Hope this helps you, now I'll go back into the shadows.