I would a pitot tube in exhaust work? There's very little 'normal' air in the exhaust, just the engine exhaust, and unless I got my physics completely wrong, will always give the same relative speed relative to the plane(pivot tube) at any given engine setting... which doesn't tell you anything about the actual airspeed (which is what you need).

Pivot tubes are 'tubes' located away from the body of the airplane for the reason that they get away from the boundary airflow to give accurate airspeed readings. Attempting to 'compensate' for other factors just introduces more uncertainty, which is exactly what you don't want when you're 5 knots above stall.