In the parcel access control tab, add a checkbox "[ ] skybox zone". If this is checked, then instead of having the access controls on the parcel applied from ground to ground + 40 meters, instead they apply to a region 768 meters below the building ceiling. the resident can put their private area in this skybox zone, with enough room (a vertical range as high as the original building limit!) to avoid being too near to their neighbor's skyboxes. As an enhancement, this could be combined with a sim to client access control over this range, such that objects completely inside a skybox zone (calculated by bounding box or other cheap measurement) you don't have access to wouldn't be visible... because they wouldn't even be downloaded to your client.