On that type of surface there was always gonna be a risk. Normal rule of thumb ofcourse is the usual 1:4 or 75 degree angle, I'd definitely go steeper on that surface as it would be like an ice rink with the early morning dew. Sand bags would be your only man there. Place a sand bag on the bottom rung of the ladder to mimic it being footed and then with the other safety precautions, 3 palmer grip, don't carry anything up, don't over extend etc, you should be fine. Youv'e been led into a false sense of complacency with your ladder mat. As you said earlier, you like to work traditional, stay traditional, dump the ladder matt and stay safe.
Neil.