I was thinking that there could be interesting and helpful events that came before werebeast attacks.
Something along the lines of the mangled corpses of drifters or travelers found by your dwarves in the wilderness. There could similar mechanics to when werebeasts are actually invading a fort, where the beast is invisible until spotted, except for a corpse somewhere on the map, preferably in a less traveled area. The corpse could just appear and be invisible until a dwarf wonders by it giving a message like "A stranger has been found ripped to pieces by some viscous creature! Best watch out for danger!". It would be more interesting if a werebeast and a traveler both were unseen on the map and the attack actually happened, giving a small chance of an actual sighting and possible small scale werebeast attack on the dwarf who happened upon it.
Another way it could work is once the pop threshold for getting werebeasts is reached one could prowl the area just outside the map boundries and attack an actual visitor, still unseen to the player, as they were leaving. Maybe when if the player watched the visitor to the edge of the map they may see it suddenly die for unknown reasons, until a dwarf happened upon the corpse. Though you'd then have to coordinate visitors movements with moon cycles. A workaround could be: a visitor leaves, then during the next full moon their corpse appears, under the role-playing presumption that they stayed in the general area after leaving the fortress, until their demise.
This could be a precursor to a normal werebeast attack, to give a player some semi-cryptic incentive to prepare for danger without really making the game easier. If they don't find the body then it's were-business as usual and the attack comes in a month or two.