So, it seems like one option would be to fly from Buenos Aires to Santiago, and then take a bus from Santiago to Pichilemu. Is that my best option?

If you fly you will pay a reciprocity fee landing at the airport in Santiago. You could cross by bus from Mendoza. From BA it is a bit long, 21 hrs. Maybe fly to Mendoza?

Once in Santiago, take an airport bus and get off at the Terminal Santiago. It is right by the Universidad de Santiago metro station. If you get the Turbus airport bus it will go into its own station which is right across the side street from the other terminal. The companies which go between Santiago and Pichilemu are Pullman del Sur and Nilahue. Check which route they are taking. The ones which go through Melipilla take less time than the ones through San Fernando, but both routes are fine. There are frequent departures during the day.

Thanks Huentetu! I had forgotten about the reciprocity fee. I'll definitely think about taking buses all the way, or at least from Mendoza to Santiago. That sounds like a good option!

Remember that there is a one way traffic system crossing by Los Libertadores because of roadworks on the Chilean side. Fortunately for you, from Argentina the buses travel by day. The buses from Chile travel by night. Expect a longer bus ride than normal because of delays and waits on the road.