Someone just sent me a reference to a funerary epigram from Thessaly mentiioning a man killed in war by a "kestros", i.e. probably a dart from a dart-sling (kestrosphendone). The context is likely the Third Macedonian War (which is, indeed, when Polybios and Livy mention the weapon in use). I'll chase up the reference.

You're right, it's good to have a direct and documentary source form the Hellenistic period (though Polybios is pretty reliable, he's only known indirectly, i.e. through quotations, for most of the period he covers). Of course, "kestros" simply means "dart" or "shoot", so it could be conceivably something else.