A terrain or symmetrical lattice mesh are the only two "objects" in which Bryce will modify polygons. i.e. Add, delete, and change the shape of existing polygons, etc. Boolean operations in Bryce do not modify any polygons; the result of the operation is calculated at render time for each individual pixel and is rendered accordingly without any real change to the true shape of any objects, or portions of, occupying that particular pixel area. There are pros and cons to this method which could be discussed forever. However, since Bryce does not alter the polygon mesh of objects created with primitives, even if you could export your prop it would look like one big jumble of geometric shapes.