If you were using CBSubs you could simply use the CBSubs Mailer plugin. However, I've no idea what subscription management you're using.

At any rate to answer your question, no. You'll need a Joomla system plugin that performs whatever check you're wanting it to check during the page rendering process (this would fire on every page load, so be careful with your usage) then do whatever action you're wanting it to do if the check is met.

It can also be done with a CB plugin, but is going to require a CB action to take place. You could for example perform your check when the after login CB trigger is fired, but check all users instead of just the user logging in. That should resolve the problem of infrequent users not receiving anything. This implementation would work even if you had only 1 active user.

CB doesn't currently have access to Joomla triggers; this will be available with CB 2.0 where we'll be providing an official system plugin to route Joomla triggers into CB triggers.