Hi @rmccunney,
I'm not aware of a way to do this for external URLs. For other content, you'd just like to the specific content item (like an assignment), and the rules of the module page would be obeyed, locking the student out if they had not reached that item yet. Because external URLs are things outside of Canvas, you wouldn't really be able to enforce module rules. I know it's not ideal, but perhaps just linking to the module from your page would be specific enough to point students in the right direction? I know it's not ideal, but I can't envision a way this could get adjusted without major changes to Canvas (which personally seem unlikely).
I hope the info helps, even if it's not what you wanted to hear.
-Chris