But what's the actual distribution of usage of emoji? I'm guessing there is a small-ish set that people actually use, just like there is a small-ish set of word

But what's the actual distribution of usage of emoji? I'm guessing there is a small-ish set that people actually use, just like there is a small-ish set of words that people will pick from if you ask them to pick a passphrase containing "random" words. Unless you pick the passphrase for them, you're not really gaining the advantage of the search space size.