In my opinion, it's always best to "take the bull" by the horns in situations like this. No point in hoping they won't notice your quant score -- they will. So your optional essay should definitely point out the quant areas in which you have performed well, both university classes and also any quant-heavy tasks you are relied upon in the workplace. The idea isn't to say "The GMAT is not accurate", but rather, "in spite of my scores, I believe I have the analytical strengths to succeed in your program" and then list your evidence for this belief.

I also suggest that if your quant scores are really below the average for these programs, you might want to voluntarily take some courses, perhaps online at a highly respected venue such as UCLA Business Extension, to show that you are serious about making sure you can handle the work. You don't have to have finished the courses by the time you submit your application, but your commitment will be noted by the admissions readers.

Good luck!
Margaret Strother