User blog comment:Xilinoc/Ch. 547 - Peace from Shadows/@comment-1268838-20130904145545/@comment-11051098-20130904175011

He probably talked to Mayuri and had him find or invent a way for Shunsui to visit the human world directly, without having to go through the Dangai or a Senkaimon.

What hint do you refer to? I don't remember any hint from Aizen about the shadows.