After reading your reply my post seemed a bit curt to me. Not my intention, actually I was very delighted that the link I found was top of the list. I hope I did not offend you.
I think you will have a difficult time finding the pH of agave after it is in the body as it's a fairly new (to my knowledge) product outside of Mexico. Without thorough research though, I don't think I'd replace the maple syrup with any other type of sweetener. I personally would hold off on doing the Master-Cleanse until I had enough maple syrup to see me through.