I've the idea to do one of these next year, there are a few that are within a day's drive of me. Right now the
Catskills SR600 is the closest and it has a lot of options for overnights. My plan is to use most of the time limit and get two solid sleeps during the nights. I think the change allowing 60h really makes these rides more enjoyable. With the right start time it should be possible to do most of the riding with some daylight.
I had a plan to try an Everest once but it was repeats up a 1000m climb, and to get to 10000m was only 9.5 climbs up the mountain and it ended up being about 250km when I mapped it out. I think a ride like that would be an easier way to get into the high rouleurs society than trying to spread it over 600km... I know for me the desire to sleep after 400km becomes pretty strong.