It might be good to keep non-ROS consumers of URDF in mind as well. Ideally they would have access to this ‘meta’ information too. Encoding it as an attribute makes that straightforward. ROS parameters do not.
It would also avoid fragmenting the necessary information to successfully parse a URDF. Fragmentation like that is common in other areas of ROS already (MoveIt fi: joint limits from urdf, but also from config yaml, can be overridden with parameters and there’s some dynamic reconfigure parts as well).
Note that other formats already support explicitly marking meshes as convex, or that capsules should be used for collision detection/physics, and they use similar approaches (attributes or a different tag/element/entity).