I think Brook already has the Poneglyph or else all the hype he got in the previous chapters will be for nothing. I can see this happening in two ways .
1) He might be hiding the rubbing inside his body, like in the below panel:2)He can summon winds from the underworld, so won’t it be cool if he can hide stuffs there and summon them later? It’s not so probable but i think it would be cool.
*Theory by White_T_poison