Detail #219: Ergative-like Topic-Comment Structure

Consider a language where the comment part is introduced by a subordinating conjunction, here presented by 'that':

man that lives in a cave
the man, he lives in a cave

However, if there is no comment, and we're just attracting the attention to a topic, the topic is also marked by this:

that man
a man, the man

However, this seems like a rather restricted thing without much real "interest" or even any way of extending it into having more of an impact on the language. Are topics and comments just too far off from anything we could turn into a system analogous to erg-abs?

I suspect they are, but I always keep second-guessing myself on it.