You might be able to use the POV files that the model viewer exports if you want to edit the models in a non-3DSMax application. They contain all the information that I managed to pull from the S3D files (geometry, texturing etc.), in fact it's those POV files that get imported by my maxscript. I've not really looked at POVRay much to be honest, but I know it's free and I know there's some sort of GUI available for it.
3DSMax is the only 3D package I know how to use with any degree of competence though, so making the viewer export other formats would be a bit painful
K.