In my spare time (not much these days!) I have been reviewing the service manual index looking for information on the HSA. I have been through it twice and there is no direct mention of the HSA. The only lead that I have is the yaw sensor that is part of the TCS. It is mounted under the hood high on the passenger side. I am not sure if this sensor handles more than yaw (fishtailing) but the calibration steps want the vehicle on a level surface which may indicate it handles pitch output too. Might be a place to start looking.
I will start looking at the brake and TCS sections in more detail to see if the HSA shows up.
2013 AGM 2SS/RS/1LE-NPP
2002 Z28 Navy Blue A4 [traded]
1996 Camaro V6 Black [traded]