Shade
Monster Junkie
Here's something that might help. The death hurler, from Dragon #309, is basically a giant siege tower. It has no melee attacks (or appendages). However, if anyone tries to climb it, they have to make an opposed grapple check or be thrown off. So there is precedent for a creature with no melee attacks making opposed grapple checks.
So, the spanner could make a touch attack (almost guaranteed success) to initiate a grapple. If it succeeds, it could make throw the creature off. Perhaps a Reflex save, with failure resulting in going over the edge, and success indicating the character is prone in its square on the spanner? I'd also state that because it lacks appendages, the spanner cannot maintain a grapple.
So, the spanner could make a touch attack (almost guaranteed success) to initiate a grapple. If it succeeds, it could make throw the creature off. Perhaps a Reflex save, with failure resulting in going over the edge, and success indicating the character is prone in its square on the spanner? I'd also state that because it lacks appendages, the spanner cannot maintain a grapple.