Jay:
The guidelines which we defined in the UK around 15 years ago are here http://www.coursemeasurement.org.uk/director-2.htm
10% max firm off road for races less than 10k. Above 10k its 500m + 5% of the race distance in excess of 10k. ie 2.6km for a marathon.
It is broadly based on the observation that off road surface can give calibrations changes up to 1%. 500m gives plenty of scope for starting and finishing a race on a playing field.
The 5% of race distance in excess of 10k provides greater margin for accuracy and also reflects a feeling that if someone is entering a road marathon he doesn't want to do a couple of miles or more on off road trails.
It just happens that this whole question is coming up for some measurement tests by a small group of measurers in two weeks time. There is a marathon with several miles on a unsealed cycle path - a former railway line, which has some rather fine but loose gravel on a compacted base. The race director wants it classified as a road race, we have refused to do so because the distance is well above our limits for accuracy. We have said no certificate - just a statement of measurement with no guarantee over the accuracy.
For the group experiment we will layout calibration courses on the unsealed path and on a nearby asphalted road and compare with several different bikes/riders/tyres.
The problem I see is one might convince oneself that this particular surface has been well enough calibrated or is sufficiently like a road that the course could be certified, but how and where can you draw a clear line between certifiable road races and cross country surface, without undertaking major calibration or surveying exercises
The advantage of the present guidelines which we issue is that it gets road races onto real sealed roads, and the question of what is a sealed road surface is usually fairly clear.
I believe a year or two ago there was a move to allow some small amount of non-sealed surface in IAAF/AIMS road races. I am not sure what the status of that proposal is. Formerly it was supposed to be all sealed roads which was found to be rather restrictive for some start/finish arrangements.