WP JSON REST API (Ryan McCue) how to query posts with specific meta data with ‘AND’ relation?

Sorry for answering my own question but it may help some other devs too.

I created this additional filter ‘json_query_var-meta_query’that returns the necessary arguments.

function adjustQrry($data){
    $args = array();
    $args['relation'] = 'AND';

    foreach ($data as $key=>$value) {
        if ( 'relation' === $key ) {
            $args['relation'] = $data['relation'];
        }
        if (  substr($key, 0, 3) === 'key' ) {
            $arg_num = substr($key, 3);
            $args[(int)$arg_num]['key'] = $value;
        }
        if (  substr($key, 0, 7) === 'compare' ) {
            $arg_num_comp = substr($key, 7);
            $args[(int)$arg_num_comp]['compare'] = $value;
        }
    }
    return $args;
}

add_filter('json_query_var-meta_query', 'adjustQrry', 10, 1);

Now, I can call JSON restful like that to mimic the Wp_query posts filter already on the server:

?filter[meta_query][key]=_newsml_categories_newsstream&filter[meta_query][key2]=homepage&filter[meta_query][relation]=AND&filter[meta_query][compare]=NOT%20EXISTS&filter[meta_query][compare2]=NOT%20EXISTS

Ref:
https://github.com/WP-API/WP-API/issues/337