A few comments:
You suggest a 24-hour ride, but don't specify a minimum mileage. Would this be a form of Saddlesore 1000? Or when you say it's a 24-hour ride, do you mean that one would ride for 24 hours regardless of mileage?
The time between sunrise and sunset varies depending upon the time of year and geographical location. So not all sunrise to sunset rides would be identical - some could be doing the ride mostly in daylight and others mostly in the dark. Also, what calculation would you use to determine when to turn around, taking into consideration your location at the time of sunrise/sunset?
What definition of "sunrise" and "sunset" would you use - astronomical, civil,or nautical?
A more macro question is this: why would the World's Toughest Riders be looking to make a ride easier, rather than harder? Most ride suggestions are of the latter type, such as suggesting a Saddlesore 1000 entirely on dirt, or entirely within a city's limits. It seems to kind of go against the grain somehow. Just my $0.02.
Ira Agins
Iron Butt Association