In fact the Product Owner need not know about it at all if you don’t want them to. In Jira, it’s not easy to have one sprint pull from two backlogs so we just put it in it’s own category called Tech Debt. That way it’s easy to filter it out (or in) by category.
Scrum for Highly Interrupted Teams

I agree with most of your article, I actually think this is just common sense to be honest. But this part… I mean I see your idea here, but in my opinion transparency is a key thing. And I am not talking about Scrum, I mean in general. Hiding work never is a good idea, NEVER. Also using 2 backlogs, how do you prioritize then