User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-1351982-20131016161206/@comment-4243147-20131016201755

I don't think the mask is a Hollow mask. When in Bankai, Toshiro usually wears parts of Hyorinmaru on his body, like the the ice claws and wings. When you look at the mask, it looks like the protrusions on Hyorinmaru's face. I think that's how his complete Bankai is supposed to look like.