Thanks for your answer.
So, it seems to me that using a wildcard to match any domain (i.e. http://*/wp-admin.php to redirect to a WordPress admin page installed on whatever domain) is not possible.
At this point, how can I solve my problem?
An idea could be to redirect to a service deployed in my network, whose only duty is to redirect in turn to the WordPress instance. Could it be acceptable, from a security perspective?