Quite impressive; I can almost see the appeal.
Had a go once. Anyone remember the old SUSS hut at Rowter Farm, Castleton? A bunch of us was staying there and it was one of those weekends when it just rained constantly and we were more or less grounded. Eventually boredom set in and, after a bit of ferreting around, we found an ancient set of skis and ski poles chucked round the back. We may have had a drink or two at the time and someone said "Has anyone ever had a go at mud ski-ing?" and, quick as a flash, we had them tied onto a pair of wellies with some baling twine. I'll leave the rest to your imagination but there's some very steep (muddy) hills in the vicinity and I seem to remember seeing a lot of sky arcing past (just like in that video). Posh Lycra was definitely not the order of the day.
So there you are - ski-ing potholer style. Bet it'd have made a much better video . . . .