In my two days of researching this, I agree that NACA ducts are not for ducting out. What you want to look at is actually a NACA cowling- but use it as a slot, instead of an entire cowling.
I havent found any wind tunnel images yet, but the air around the wheel would be pretty ridiculously turbulent, making any duct less effective. If you want to vent air from the wheel well with the most power, I would suggest putting a vent
in front of the well, where the air flow is
more stable.
I'm just in civil engineering though, so if I find something else out then I'll post back here.