im persistent... LOL
how about in a hot process? in which there will be no lye available in the "cooked stage" of the soap...
Yeah, you're very maqulit! Haha
When lye reacts with oils, it forms a salt that's basic (alkaline) by nature. That's soap and it doesn't matter if it's cold process, hot process, or rebatch. You can't change its pH by adding something acidic, otherwise it wouldn't be soap anymore and this is exactly what will happen. Mixing it with the acidic additives enough to get the effect you want in hot process would probably turn it into a semi-liquid mess that will never solidify because it wont be soap anymore. It's called an exchange reaction, and you'll turn a lot of the fatty acid salts back into fatty acids again. I'm not a chemist, but it's something like that.
Since you have your heart set on making a solid whitening bar, do the syndet bars. I'm pretty sure it's the only way to get a solid effective whitening soap.