It really seems like we should be adding some acid to our mash to ensure the PH is correct when using BIAB.
Unfortunately, mash pH is a complicated subject and I think you are missing some of the more important concepts. First, mash pH is not dependent on the volume of the mash. Woodlandbrew states it nicely:
"Because it is the grain driving the strength of the buffer, the pH change of a volume of acid will not be effected by the amount of water in your mash tun. Meaning that whether you have 2 gallons or 20 gallons of water, it will not affect how much 1mL of acid changes the pH."
It is important to understand that pH is on a logarithmic scale, so changes in water volume have relatively little effect on the pH of a solution.
Secondly, initial mash pH is determined by your water chemistry (ie. residual alkalinity and mineral content) and your grain bill (types and weights of grains used). You can get pretty darn close to a perfect mash pH if you managed these two parameters. Adding acids to the mash should be only be done on an as-needed basis if you cannot hit your initial mash pH by adjusting your water chemistry. That is the intent of the multitude of water chemistry spreadsheets available online. Brewing using the BIAB method doesn't generally change the grain bill, so there is no reason to assume that acid should added to all BIAB brews.
Assuming that proper mash pH is established through by balancing water chemistry and the grain bill, then I think the only real consideration for BIAB is whether to acidify the sparge water to reduce tannin extraction. I do a no-sparge BIAB with full volume mash and so far my mash pH has always hit 5.2-5.4 without acid additions, even for lighter beers like blondes and pale ales.
For some additional reading on mash pH, check out
Chapter 15 - Understanding the Mash pH in How to Brew and additional discussion of
no sparge pH in this thread. The
brewing water chemistry primer thread is also a great resource.
