I'm pretty sure that disabling Stabilitrak was mentioned earlier in this thread, though it didn't seem to attract further discussion at the time. Let us know how it turns out.
It does make sense to piggyback hill-assist on top of whatever stability control system is already in place, as stability control requires at least one inertia sensor. Adding another one to detect significant longitudinal slope into the same little box, another wire or three, and a few more lines of code shouldn't be difficult.
The real gem in that linked thread is that there's hope that a way to reset the default to 'off' exists.