Well....the clay doesn't really shrink so I think you are safe there, but a good point none the less. I would be more worried about the adhesion. Polymer and wood don't always play well together. Since wood expands and contracts some the clay tends to loosen itself over time. On the other hand there are some polymer artist out there that cover wood furniture with veneers of polymer. The difference with this being the clay is cured before the lamination.
The second issue I would worry about would be getting all the voids (CSue). Polymer is much different then PR or aluminite with the later two being able to pour and fill every nook and cranny. Polymer clay would leave voids and air pockets that might cause weakening down the road.
With all of that said, try it. Maybe it will work?