I found the HSA in the manual. It was listed in the ABS section for cars with F55 (ZL1 shocks option code). My post about the yaw sensor was correct. It is described as a multi-axis acceleration sensor which interfaces with the EBCM.
One thing that I did find is a note under the hill start assist section: "Note: The Hill start assist feature will not operate if the stability control is disabled." We should try to turn off the traction control/stabilitrak systems and see if the HSA is defeated.
2013 AGM 2SS/RS/1LE-NPP
2002 Z28 Navy Blue A4 [traded]
1996 Camaro V6 Black [traded]