How to apply pre_get_posts to a custom query?

For some use cases, couldn’t you use $query->get() to target specific custom queries?

For example: say you’ve got a custom query for a slider post-type. Perhaps something like this?

function wpse64950_filter_pre_get_posts( $query ) {
    if ( ! is_main_query() ) {
        if ( 'slider' == $query->get( 'post-type' ) ) {
            // This is a query of slider posts,
            // and isn't the main query;
            // Do something
        }
    }
    return;
}
add_filter( 'pre_get_posts', 'wpse64950_filter_pre_get_posts' );

Granted, this will target every such slider post-type custom query; so it won’t work in every case. But it should work in some cases.