My service would essentially be to offer a tiebreaker to choose the more legitimate of two competing blocks, each hint I sign would be open to scrutiny.  If I start publishing crap (for example, I am favoring a revision of a block that contains an obvious double spend against an earlier revision of that same block, or condemning a block with perfectly valid transactions, or am attempting to roll back several blocks at the same time without a flamingly obvious well-known good reason), people would see this, they would ignore me and go elsewhere for the same service.