经过进一步研究，我想出了一个解决办法。鉴于以下情况WP_Query:

<?php

new WP_Query(
    array(

        \'s\' => \'some keyword\', //keyword criterion
        \'category\' => \'some category\', //taxonomy criterion

        \'_spec\' => \'some value\', //custom criterion to be handled by the plugin

    )
);

?>

<?php

add_action( \'pre_get_posts\', \'my_pre_get_posts\' ); //hook into the query before it is executed

$custom_where_string = \'\'; //used to save the generated where string between filter functions

function my_pre_get_posts( $query )
{
    global $custom_where_string;

    //if the custom parameter is used
    if(isset($query->query_vars[\'_spec\'])){

        //here you can parse the contents of $query->query_vars[\'_spec\'] to modify the query
        //even the first WHERE starts with AND, because WP adds a "WHERE 1=1" in front of every WHERE section
        $custom_where_string = \'AND ...\';

        //only if the custom parameter is used, hook into the generation of the query
        add_filter(\'posts_where\', \'my_posts_where\'));
    }
}

function my_posts_where( $where )
{
    global $custom_where_string;

    //append our custom where expression(s)
    $where .= $custom_where_string;

    //clean up to avoid unexpected things on other queries
    remove_filter(\'posts_where\', \'my_posts_where\'));
    $custom_where_string = \'\';

    return $where;
}


?>