Since no one has bitten yet, I will try to answer quickly and confirm/find sources latter.
According to those opinions, such as Rav Moshe Feinstein zt'l, who do no require a mechitza to obscure the view of the women's section one might think that no additional barrier is required on a balcony beyond what the ma'akeh needed for safety. Nevertheless Rav Moshe writes that insofar as [and perhaps only in places where] women are prone to not cover their arms according to halachah it is necessary for their to be a curtain high enough to conceal this. (Igros Moshe O.C. 1:42) (The Igros Moshe does note that this isn't always done in practice and attributes it to the pious being silent in the face of indifference or opposition, and requires the men to turn away during prayer. I am not aware of any publish psak that allows a balcony without a curtain lechatchilla in such a situation.)
According to those opinions (such as Maharam Shik, O.C. 77, Tzitz Eliezer 7:8) who see the role of a mechitza to obscure the view of the women's section a balcony alone would not seem to suffice and a partition would also be needed.