Just hook the harpoon up to a spool of endless rope to swing more distance, and the rules are that if you make a Strength check, you can control the movement of the target you've 'pooned - I'd say that's dragging 'em, though how far is your DM's call.
Another fun one I like is to stick an Immovable Rod either IN the harpoon or on the other end of the rope. Stick it and click it - instant dog on a chain!