I wonder what experiences measurers have had with measuring off-road courses repeatedly? I have in fact been quite surprised to find very consistent results over some of these non-standard paths. That would include DC's main cross-country venue, with a lot of paths through the woods, and other parts of the course defined by "key points" that are easy to lay out. Also a number of courses along the C&O Canal, a mixed dirt and gravel surface;and courses in various parks in the area. Based on my experience I would side with Jay, that we are justified in going ahead and measuring such courses,IF we can lay them out in a repeatable way.
As to Duane's point about calibration:I think it's a good idea to lay out a cal course on comparable surface. I've done this for the C&O towpath. But if it's not convenient I don't think that's a deal-breaker.