I've have heard of geologists using bleach to clean certain key outcrops. There is no danger of damaging/bleaching granite; you need much nastier stuff to affect silicate minerals. Don't use it on limestone or marble however, you will dissolve it. As for the environmental effects; I agree will Fred, probably negligible after the first rainstorm, but then I work in the mining industry and have slightly skewed perspective
I can't speak to the possible carcinogenic effects however, sounds nasty. If you pour HCl on certain sulphide minerals, (eg. pyrite or fools gold, pretty common in many granites) you get toxic chlorine gas.
All things considered, I'd stick to the wire brush. I won't want to accidentally get bleach on my climbing gear either.
I have heard of guys in Squamish (70's) pouring gasoline down vegetated cracks and lighting it on fire. This seems to me to be a much safer and more sensible option. Do in the fall when the leaves are dry, it'll burn better