Dear
@Quilter99755, I do not want to cause undue alarm for you or your daughter, but would be remiss if I did not mention a concern about using rice that has not been sufficiently treated to destroy Bacillus cereus spores, which is very hard to do because they are extremely hardy, heat resistance and survive a variety of adverse conditions that normally destroy other micro organisms. It does not survive pH extremes well (very low pH and very high pH) nor low water activity (a formula I am not good at calculating) however, so perhaps the formula you are making is safe, but as a retired nurse, I would be very careful to thoroughly research AND probably consult an epidemiology and infection control specialist to ensure that the product is truly safe on the cochlear implant area.
I cannot find sufficient information on the process used to produce rice powder that would give me any indication that the spores would be destroyed or absent in the resulting rice powder, so I would not likely add rice powder to a product with a pH higher than 4.5 (and only if I had professionally accurate equipment for ascertaining the true pH level of said product).
Perhaps the 11% water content is low enough, but I really am unsure if it is or is not sufficient to foster growth of B. cereus spores, and once water is added during the shampooing process, the growth of B. cereus spores is more likely if they are present, so I would worry about long term use of a syndet bar with added rice.
I am no good at calculating 'water activity', and would not expect to own the equipment necessary for accurately testing, so that's another aspect that concerns me. However, in link #3 below, the 'water activity' level of shampoos (not sure how much it may vary) is sufficiently high to foster growth of B. cereus. The listing may only be for liquid shampoos, though, and may or may not encompass syndet shampoo bars. And of course I would suspect a particular formula would play a part in the actual measurement of 'water activity'.
Due to the cost of proper testing equipment, perhaps Challenge Testing would be a safe route to determine if this formula prevents growth of microbials that can cause infection. Or perhaps there is an antimicrobial cosmetics additive that prevents B. cereus spore growth (I have not researched all of them, so do not know.)
Perhaps I sound like an alarmist, and I am sorry if I am coming across as such. If you have already done the research or consulted your daughter's physician, please forgive me.
Below are some references regarding Bacillus cereus which refer to its veracity in the environment and its ability to survive and cause infection in humans. The first, second & third links are related to personal care products specifically; the others are more general, and some more related to food, although one other is specifically related to a skin infection caused by the organism. Some are rather technical, but the bottom line is that this organism is extremely persistent and quite capable of surviving normal decontamination methods and causing infection in human skin.
https://onlinelibrary.wiley.com/doi/full/10.1111/ics.12191https://www.cosmeticsandtoiletries.com/research/methods-tools/article/21836223/water-activityhttps://www.cosmeticsandtoiletries....ols/article/21836223/water-activity#235095241https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7913059/https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0194346https://wchh.onlinelibrary.wiley.com/doi/pdf/10.1002/pdi.1950https://tinyurl.com/5n8y6u6x (opens a pdf)