The simplest solution is to amend the entry form for Audax >200 km so that the rider has to confirm that they will not drive for 8 hours after the event.
I'm not a fan of things like this, since it assumes things that may not be true for everyone, and has an unwanted effect on those that it shouldn't apply. There are many riders that can be finished with a 300 within 13 hours, which is usually before 7pm - I'm not really willing to state that I'll wait around for 8 hours (by which point driving might be
more dangerous) when it might only be an hour's drive home.
The action that Lightning Phil says he's taken by "nudging" his riders to consider adequate rest seems like a good way to go and seems like something people will pay more attention to than a tick box on a form. I've had an email this week from an organiser about a forthcoming 400, so perhaps this thread has had a good effect in that way (unless they were planning to do that anyway).