For posts (Posts, Pages and custom post types), the very first parameter for register_meta() is always post and to set a custom post type, use the object_subtype argument in the third parameter like so:
$meta_args = array(
'object_subtype' => 'splash_location', // the post type
'type' => 'string',
'description' => 'A meta ...
WP_Post has a property post_parent by default. Usually this is used for parent-child relationship on hierarchical post types, such as pages, but nothing actually prevents you from linking two different post type posts together with it.
You'll just need a metabox on your travel report CPT, which allows you to select an author CPT post (id) as the parent. This ...