I've seen this problem on a (human) model too, but didn't find how to fix it yet.
As it appears only in SunFlow, it's either a bug in the code I wrote that transforms Java3D shapes to SunFlow shapes, or a bug in SunFlow.

It is a shame all Google Sketchup human figures are very low poly

At least these models exist, and Sweet Home 3D users can use them even if Google 3D Warehouse models are not distributed under a free license. Personally, I found them quite nice, and I appreciate that many models in Google 3D Warehouse use a medium resolution. Most Sweet Home 3D users don't need 3D models with too much details.

I don't have time to try this at the moment but I guess the only way around that would be to export them as .obj files, smooth them then import them again? Don't want to cause any problems with the model designers though.

I tested it and it didn't change anything. [:(]