Well in the rules, the only conditions for a work to be eligible is to be CG. Then if you do 2D, 3D, matte painting or whatever, it’s not your tools that will be congratulate, but your final result. (I’m working in 3D too).
So, maybe should you take advantage of 3D over 2D like easy duplication (which is a great advantage if you are planning to have a lot of vegetation), ambient occlusion (2D artiste have to paint it!), and lightning which will be computed for you (and which is pretty simple in this case).
So my point of view is, as long as your tool are CG, only the final picture will be considered…so go now and put your polygons together!
(PS: for the WIP, it has been said that it is the standard wip section of the site, so maybe just put a mention like “[nvidia sureal contest] blablablablabla”)