So if I'm understanding the transmog system right, if a cloth BOP item that I haven't learned yet drops while on my hunter, as long as it's not class specific (like a tier item or whatever), it'll be unlocked for my alt mages/warlocks etc?
No, it has to be looted on the character that can use it. So if your Mage looted it, your warlock could use it because they use the same item types.