Drop-in files, like advanced-cache.php or object-cache.php, sit in the root of the wp-content directory.
Is it possible to give drop-ins in their own subdirectory of wp-content instead?
_get_dropins() in wp-admin/includes/plugins.php there are no filters to set the path to the drop-in files. Also, looking at
wp_plugin_directory_constants() in wp-includes/default-constants.php there are no constants set specific to drop-ins (as of 3.8.1).