You're right, your quant score is a bit low. However, with your background, there is unlikely to be any doubt that you can handle the coursework. You'll need to address this in an optional essay for each school (HBS doesn't give you much space, but the other schools do), that simply acknowledges that your GMAT quant score is a bit low, but that you believe you have the skill set to do the work, based on ... list your undergraduate coursework, quant-heavy projects, etc. Your goal is not to snow the adcom or to state that the GMAT is irrelevant; instead, you simply acknowledge that you know you have an issue, show what you have done that demonstrates your quant skills, and hope that this will be enough. In most cases, the schools will be able to make an accurate assessment based on your transcripts and work history, so you're just letting them know that you acknowledge your responsibility here. See what I mean?

Good luck,
Margaret Strother