At the risk of not adding much to the discussion, I'm not sure the issue is slippage. Personally I think the front tire deforms differently on softer surfaces than it does on the hard surfaces on which we are directed to calibrate.
The question comes up "how hard is hard", and the answer is that I'm not sure. One of my experiences on a crushed limestone path on a a day when the temperature flirted with the freezing mark tells me that the same surface can be hard on the first ride and soft on the second, an hour or so apart. Another experience measuring with two measurers gave me different numbers on a dry crushed limestone path but almost identical numbers on the paved portion of that path, on the same measurement ride. As Duane states, you'd almost have to calibrate on every different kind of surface the course traversed. By the time you did that, measuring each of those with a steel tape would almost be more efficient.
If I remember correctly, Mike Sandford offered in an earlier post the in the UK there's a limit as to how much of the course can be on an unpaved surface and a bicycle measurement still be accepted. Maybe we should use that as a starting point.
I would also opine that if a cross-country course can be documented the same way that we document road courses, and it's measured with a steel tape, there's no reason we shouldn't certify it.