i don't recommend it. first of all, as sad as I am to say it, schools that definitely won't let you in b/c of your MCAT scores probably won't read your PS. You'll make the filtered category and will get a "We would like to thank you for applying to the class of 2007, unfortunately..."
if they aren't sure b/c of your mcat and want to see how you are as a person, they'll read your PS, and if you spend it talking about your MCAT, you aren't convincing anyone. Essentially, you're saying "take my word for it, I can do better" but you aren't offering any proof. Instead you should want them thinking, "her MCAT's are low, but she sounds like our kind of student, let's bring her in for an interview and see". you gotta learn to let it go and be yourself, and if you think you've got a good essay with MCATs in there, then send it. in a few years you are going to have some big decisions to make about people's lives and won't always have someone else to ask, so here's one for you to answer about your own.