No, you cannot.
Bracers of armor aren’t a “type of armor, such as chain shirt or splint mail,” as Armor Focus requires you to select. They are, as a pair, a wondrous item whose magical property is to apply an armor bonus to AC, but that isn’t the same as being a type of armor.
Importantly, someone wearing bracers of armor and not wearing other armor is still considered unarmored—that is the point of the item, after all, since they exist for the sake of monks and other unarmored characters. All other characters are vastly better off with real armor. And Armor Focus doesn’t allow you to choose “not” as your “type of armor.”
But it’s not a balance concern.
There’s no really important reason balance reason to bar people from picking “unarmored” for Armor Focus, and allow them to receive that bonus as long as they aren’t wearing armor—including when wearing bracers of armor.
That said, the only really meaningful benefit of Armor Focus is the ability to qualify for armor mastery feats without armor training. The AC bonus is pitifully small, and armor bonus to AC is the worst defense in the game. That is never worth a feat, for any character. You should only take Armor Focus when you need to in order to qualify for some very good armor mastery feat. (I don’t know if any of them are all that good, but there are too many of them for me to categorically say none are.) The problem with this is that a lot of armor mastery feats do different things for different types of armor—and don’t have a listed benefit for unarmored characters. That makes it very unlikely that Armor Focus is a good choice for an unarmored character, even if a houserule says they’re allowed to take it.