I don't really think there's a way for any of the players to send signals to their respective allies. I feel like that'd be too easy. On the other hand, without any signals, I agree, how can you even begin to deduce anything. I remember the moments with Baku and Kyara, regarding those "signals". I'm not sure if that will end up being relevant but it just came to mind.
But yes, you're right. Just as Baku figured out Lalo's remaining cards are from his own, I'm sure once Hal gets to know what Baku's cards are, he can create a strategy for it. At the very least, if Baku is somehow able to let Hal know what his cards are, then that's game over.
Regarding Baku pounding the table, in chapter 445, Baku shouted all that shit for some reason but I can't see anything that will end up being useful for Hal. After all, it's only in that moment that there is a path open between the players below and the players above. But still, even in that situation, Hal and Fukurou were inside those contraptions anyway. I doubt they're able to hear anything, especially Hal since he was being shanked at that moment.