From the top position you can hit the Go Go Plata as Shinya Aoki proved in competition at Dream. The technique requires some flexibility but if you can sit in a chair and cross your legs so that your shin is paralell to the ground then you have enough flexibility to do the technique.
In order to pull this technique off you must maintain good base.
In the video you can see as Eddie teaches the technique to Shinya that he posts his arm on the same side that he does the Go Go Plata.
If you do not post that arm to maintain your base you will be rolled over to your back. You may be able to the Go Go Plata from the bottom but your opponent's weight will be working against you instead of your weight working against him from the top. The point is maintain your base.
Another way to finish the technique is once your foot is across his neck you put both hands behind his head and pull up for the submission.
