This compound particle has a similar effect as to say I heard someone say something. ばよ in だってばよ doesn't play a key role. Often, だって suffices. だって or だってばよ can be applied to direct or indirect narration. For example,
あなたの愛している人が、「直美ってかわいい。」だって（ばよ）。 I heard someone you love say, "Naomi is cute."
It can be said that ばよ is added to emphasize, like I'm saying I heard someone you love say, "Naomi is cute." This is sometimes the case. At other times, ばよ means nothing after all.
Another possibility of the use of だってばよ is to emphasize one's assertion with だってば and with a よ, functioning almost like don't you hear me. For example,
B: We MUST HURRY, don't you hear me?
However, quite often in comic strips, such postpositional particles are meaninglessly overused as such that the grammar is violated. I suspect that your reference to だってばよ applies to this case. It could mean nothing as you mention.
And, "Yが、Zだって" suffices to understand that "Xは" and "聞いた" are there even though they're hidden. If you leave only one of "Xは" or "聞いた", it's no good. You need to take out both and leave only Yが、"Z"だって.
However, I think it's not a good idea to go into this analysis deeply. After all, だってばよ is just a compound article which doesn't exist in English. I tried to capture what だってばよ means using an English pronoun, verb, etc. as a whole sentence, not word for word. The sentence structures are radically different between the Japanese version and the English version.