How to communicate with the users of an extension?
The short answer is no.

Per this blog post, only the core team can control in-app messages.  I'd say you'd have to make your appeal directly to them.  I'm sure they'd be hesitant (with good reason), but you may be able to make your case.

However, that wouldn't do any good in your case.  I've just skimmed CRM/Core/CommunityMessages.php.  The pick method allows message display to be controlled by the permission of the logged-in user and by the components that are enabled (e.g. CiviMail) - but not by extension.