Thanks for your useful post okh [:)]

The more I think about it, the more I like the idea of adding a mention similar to the following one (including the links) to documents presenting a project made with Sweet Home 3D:

Project designed with Sweet Home 3D. Includes 3D models distributed under a free license.

The good thing with providing a link to the free models is that people can identify each model if necessary to find its author and license.
Of course, if the designer included 3D models coming from other web sites, it will be up to him to adapt the previous note.