What a good idea! Some folks will be able to find this in the bulk aisle at their local store as well (or in a cosmetics section, etc). Any idea what ratio would be needed to get same acidity level?
FWIW, I found this resource on using citric acid for household uses.
I used a heaping teaspoon in a quart of water and let it sit for about an hour before I used it. I washed my hair with my homemade soap http://www.goingzerowaste.com/blog/how-to-make-a-shampoo-bar , rinsed and then put the citric acid water mix in a big mixing bowl and put my long hair into the bowl and splashed the mix all over my head. Then I rinsed with water. Easy. Hair is very soft, but not coated soft like you get with cream rinse or other products.