The thought that it might be Haruka's crossed my mind as well. Though that would mean that it was in the shop and that Yuuko would have met him before as a customer...which I don't believe is possible since Yuuko and Haruka said something along the lines of "we finally meet" in one chapter close to her end, where she i guess finally entered the dream wold.

Though...that brings to mind exactly why it is that grandpa sit outside Yuuko's shop in Watanuki's dreams and not some other location, maybe he was involved with the shop pre-yuuko.