"the original author of a prop has copyright over the mesh..."

...and no one else has any right to use this copyrighted mesh in their own work without the author's permission.

If he so chooses, the author may grant you the "limited" right to use his original work in your renders including any kind of restrictions he wishes to impose. These terms would comprise a binding agreement like any contract. If you do not agree to the author's terms, regardless of how silly you may consider them, then the right to use the author's copyrighted work is not conveyed to you. His work, his terms, his call, never the end user's.

As your new work would be a derivative of his original work, any subsequent use of your copyrighted image would be bound by the terms of the original agreement. No?