If your A8 is a D3 model, there should be a hex headed tool in the toolkit on the bootlid. This will enable you to manually release the rear calipers on each side. Its quite fiddly and easier done with a rear wheel removed first to help with access. The Hill shouldn't be an issue with the transmission in P.
If the tool is missing, a set of standard hex type bits that come with battery drills etc. will work instead.
I had to do it one morning when my left epb motor failed in the "on" position.
Got me mobile until I could get it booked in for replacement.
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