client_damage does not hook fall damage? I'm surprised to hear that..have you tried?
Anyway, another method would be to use hamsandwich..
RegisterHam(Ham_TakeDamage, "player", "hurt_hook")
goes in plugin_init
then make a function below..
public hurt_hook(id, idinflictor, idattacker, Float:damage, damagebits)
idinflictor would be 0 for ground (that bastard named world always hurtin us)