Hello! You can control who SEES an event based on category access level. I'd recommend setting the public / registered / special access level for the event's category to something where only your test users can access when they login. Then you can publish the event and the public won't see it. Hopefully that will help!
Thanks for the kind words. We've put years of development into DT Register and it's nice to see when people recognize the efforts
Don't forget to tell people in a Joomla Extensions Directory (JED) review about it too!