Hill Assist Control (HAC) is for automatic transmissions. It was designed to help start up from a slippery surface on a steep incline. The manual transmission comes with a clutch start cancel mode for similer scenarios. My Tacoma has this feature as well, but I have never used it and am not sure how to. The new BMW 3 series comes with a system that holds the rear brakes for up to 3 seconds to prevent the vehicle from rolling back on an incline. I think you are looking for something like that, I have not heard of any aftermarket add on like that though.
