I feel like ghosts don't dream. I have no idea what it was in Wraith, but yeah - I'd agree with you there. That does leave Sandmen a bit in the hole, though ... can't do much to ghosts, and can only affect sleeping mortals.
Maybe some of the illusion powers of the Haunters get moved over to Sandmen, and Haunters are more about possessing objects and places and producing physical effects (could even merge them with the machine-possessing playbook)? Then, Sandmen can pull stuff out of dreams and nightmares, creating illusions perceptible to ghosts and waking mortals.