Looks like the pad did protect your arse from worse injury.
Less than it looks, as the damage went down the left side of the pad somewhat, and the pad mostly protects non-sticky-outy-bits. As far as I can work out, after the right elbow impact I must have been pulling myself up onto the sliding bike with my left arm, which is why that side of my arse took the brunt of the abrasion, and my right leg and shoulder are completely uninjured.
There's a low level ache from my right hip that suggests some unnatural contortion at some point. I also have no memory of unclipping.