How would someone born in 2175 plausibly be exposed to sufficient eezo in the womb to develop the nodes necessary for biotics? Said person would be born on Shanxi to a middle-class naval family.

Hm ... well it depends; was he born on Shanxi necessarily or might that be edited slightly? If you make Shanxi his home but not necessarily where he was born, one could say he was born aboard a starship on route to Shanxi which came under assault from pirates or something, and the engines were damaged resulting in a leak in eezo?

Just one idea.

Drugs work, as well. You know...drugs? Red sand and the like. Only if you're willing to make his/her Mother a hardcore junkie though I'm doubting that's an option. Err, never mind. *sniff*