On "camping" trips, we carry our Cabrio in the garage of our motorcoach. We load the Cabrio up the rather tall ramp, backwards. The car has no problem backing up the steep ramp even when it's wet. To have good control backing up a slippery slope, it helps to use the hand brake as needed. If you start and stop and start again you can get the goofy creep thing going, so it helps to use a bit of throttle and control the precision of the backing with the hand brake.
It IS quite possible to get a bit of wheel spin on a real slippery surface, like wet steel for instance. At idle throttle position the ESP will pretty much keep the wheels from wanting to spin hardly at all, but you can "induce" some spin when needed by using a bit of throttle and then you can control it with the hand brake. Sounds goofy, but sometimes a little wheel spin is a good thing and will get ya going just when ya need it.
just a thunk