Pull GET parameter from URL in functions.php

Contact Form 7 provides a recipe for Validation as a Filter which looks very much like what you are trying to do.

Adapting their example to your code, I believe the following should work for you:

add_filter( 'wpcf7_validate_text*', 'custom_password_reset_validation_filter', 10, 2 );
function custom_password_reset_validation_filter( $result, $tag ) {        
    if ('token'==$tag->name){
        $token = isset($_POST['token']) ? $_POST['token'] : "fk";
        write_log("TKN: {$token}");
    }

    return $result;
}