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;
}