There is a way to put a unique ID in a model that will not go away when it is converted to another format. Simply attach a very small group of vertices in a distinct pattern in an obscure area of the model (for example, in the toe). Under normal magnification it will appear to be a single point, but when zoomed, the pattern will be obvious. If you didn't want a pattern, you could even do your name in vertices. It would add more bulk to the models size, but it is an elegant way to "watermark" a model. If Renderosity would be willing to keep an archive of patterns, it would even be possible to "register" them. Of course they would have to be sure your pattern does not match any submitted to approve the "registration" and they would either have to not give anyone access to the archive or password protect each individual "registered patterns".