Pete's absolutely right that due to their nature describing most cross country courses is very difficult. However, I see a need for an accurate measurement, particularly when times are often compared from course to course. Obviously, how "fast" a course is dependent on terrain too plus the level of competition. That aside, there does not seem to be any standard. We have some courses locally that are claimed to be 3 miles but when the posted times are a couple of minutes faster than on other courses of comparable terrain (and competition), there's a disconnect somewhere.
Maybe a "temporary" e.g. a 30 day (or less) certificate could be considered for cross-country courses if they are measured in the days leading upto a race where temporary barriers and/or chalk/paint marks are placed to identify the course route.