If there is G-amylase in barley, I would suspect that the rest to optimize its activity would make a very dry beer. From what I can tell, this is the enzyme most responsible for hydrolyzing the starches in rice for sake making. A pH of 3 may do more harm than good as you may denature some other enzymes in your mash. The temperature would also favor proteases, meaning you would be left with little in the way of head retention and body. All that being said, if you decide to try it I would love to hear about the results. As a chemist, I appreciate those who experiment. To adjust pH, I would probably use a mineral acid like phosphoric as it shouldn't impact flavor too much and would require minimal addition. Then, you could add potassium or sodium phosphate to bring it up.