Two questions out of curiosity: Couldn't you just use very diluted pot ferri and have some running water to play on the bleached bits once you get to where you want to be and then re-fix?
How does the pot ferri and pot bromide work that's different from what I said above?
This is the technique that Bruce Barbaum teaches in his workshops. He uses a very soft haired brush such as the ones used for Chinese calligraphy and a very dilute Pot Ferri and of course a small hose of gently running water. It gives a very controllable way of bleaching but like all bleaching it takes practice to keep it from getting out of hand. With this technique it also helps to dip it in the fixer periodically so you know where you are at.
No escaping it!
I must step on fallen leaves
to take this path