I rode a 300 on Friday and after 6 hours in the cold dark and rain got back after the last possible control point, the pub, had shut. I decided I did not want to continue to the next available town for an alternative final control which was another 15km further on, plus the 15km home again after, even if the organiser had decided to accept an alternative finish to the one he was told of. Call me soft, but I didn't need the points and I knew I had done the distance (actually 330) which was good enough for me.
That said, it would have been nice to have the points.
If a postcard used to be acceptable as proof of passage, why not a picture of the village sign, say, taken with an android mobile and sent to the organiser by email at the time? It would be date and time stamped both on the photo file and the email so would be reasonable proof of passage.
Just an idea.