"...why would it make a difference adding the citric acid to hp after the cook?..."
I should have qualified this; I was tired after a long day and didn't think things through as carefully as I usually do when I write about stuff like this. My apologies.
There are covalent molecules and ionic molecules. A fat molecule is pretty much covalent, meaning it would prefer to remain stable and intact for the most part without a lot of coaxing. A sodium hydroxide molecule is strongly ionic, meaning it will split apart into Na+ (sodium ion) and OH- (hydroxide ion) given only the tiniest bit of encouragement. A potassium hydroxide molecule is likewise strongly ionic. Soap is somewhere in between -- it has some ionic and some covalent characteristics from both of its parents.
The sodium (or potassium) end of a soap molecule is the ionic part. Given the right conditions, this ion will unhitch itself from the fatty acid part of a soap molecule fairly easily. The old soap makers knew this -- they would make a harder soap out of a soft KOH soap by adding lots of salt (NaCl, table salt) to a simmering water-and-soap solution and let it cook for awhile. Some of the Na+ from the salt would replace the K+ to make a harder sodium soap. The point to keep in mind is that this replacement reaction is not complete -- only some of the finished soap ends up being sodium soap. If the soapmaker wanted a 100% sodium soap, then they had to start with a sodium lye, not potassium lye.
(edit) Adding an acid to a finished soap is somewhat different than adding the same acid to a saponifying soap batter. The soap has to break apart first before the errant sodium ion can react with the acid, and that takes a bit more encouragement due to the more covalent nature of soap. (end edit) Adding acid to a finished soap can cause the soap to un-saponify to some degree given the right conditions. The rate of this un-saponification reaction depends on the strength of the acid, the amount added, as well as other variables such as time, water content, temperature, pressure, etc. Pure fatty acids are produced in industry and the lab by making a soap and then unsaponifying it with a strong acid such as nitric, hydrochloric, or sulfuric. When the resulting salt is washed out of the mixture -- voila -- you end up with just the fatty acids.
The success of adding an acid to an HP soap without altering the soap ~too much~ will depend on the type of acid (a weak acid is better), the amount added (less is better), the water content (lower is better), and the temperature (cooler is better). The reason why a soapmaker could have reasonable success with adding some citric acid to a solid soap but have a failure adding it to a liquid soap is likely the different water content of the two.
Okay, that said, would I personally add an acid to a shampoo soap? Nope.
Even though I think adding a ~small~ amount of citric acid to a finished solid soap might work okay in the short term, I would be concerned about gradual degradation over time. If one wants to use a shampoo bar made with lye soap, then one needs to quit expecting one product to do everything -- clean, lower the pH, and condition -- and instead look at using an acidic rinse with or without a separate conditioner.
After I shampoo with a 'poo bar, I squeeze out a dab of homemade conditioner, mix a small pinch of citric acid into the conditioner, work the mixture into my hair, leave on for 15-30 seconds, and rinse. A citric or vinegar rinse with just water didn't work well for my fine, wavy hair, but this mixture does. If a low pH in shampoo is an absolute requirement, then use surfactants with a lower intrinsic pH. They will be more chemically stable and reliable.
So now I've turned what most people want to be a black-and-white issue into a big blob of varying shades of gray. Chemistry is that way ... a lot.
