It would appear that the Joanna Dark Figurehead is being given away at SDCC this year. If you are over there, be on the lookout for her!
Don't forget, the in-game DLC given out at Comic-Con is a time-limited exclusive for 60 days. Rare will make it available for everyone to acquire in game after that period! Also, as @Musicmee pointed out, it is also available if you purchase anything from the Sea of Thieves store until midnight Sunday 22nd July BST.