My two cents.
MPP/MPA can be really heavy in quant, especially stats and economics (Harris told me this on my visit). I don't think they'll fault you too much if you can come up with a solid explanation as to why your GPA was low and why it shouldn't affect your admission decision (that's what I did).
I took two stats courses, one calc, one finite, and at least five other quant heavy courses in undergrad, but what a lot of these programs are is stats and econ. Learn how to run regressions (it really isn't too bad, plus you scored well on the quant section of the GRE), how to data mine, and how to interpret results and draw conclusions. If you have written any papers for stats or any quantitative course, use that for writing samples. I personally used two stock write-up's from my investing courses and a 20-page paper I had for my stats: decision making course.
You seem to have a lot of experience in the field too which can also really help make up for your low GPA and you have strong GRE scores. Spend a lot of time on your essay, as I think that was what pushed me over the edge.