In the local races around here, and I suspect this is true of local races in any place that CX is fun, the organizers really try hard not to have to pull people. Ryan Trebon and Cody Peterson showed up for the first race of the year last season and lapped everyone else in the A race. It would have been quite a scene for all of them to be pulled. In the beginners race that day there was a guy who finished three laps down. (It was a tough course.)

As long as your out there giving it your best effort and you hold a predictable line when faster riders come by, I doubt anyone is going to complain.

Definitely let the scorers know if you have to DNF. They'd probably figure it out anyway, but it makes their job easier if you tell them, and the staff who make races possible are certainly worthy of any consideration we can give them. Alternatively, pick up your bike and run it out. I saw a guy do two laps that way this year. I was very impressed.