This is all good info. Thanks, guys.
To recap: It sounds like I should require pre-registration before this event (usually not my personal style) so I can get a sense of who is really committed to this marathon. To be humane, I could let people sit out one round of their choice (to nap or relax. I may want to add a couple of forced breaks in there somewhere for these reasons.) Otherwise, like a '50s dance marathon, I could say that when you drop out you stay out!
For this tournament to have maximum potential, I should strive to pre-register in multiples of 4. (In other words, open it up to 16 players, then increase to 20, then to 24, etc.).
Finally, the Group Match Play results at the end of the 24 hours would be the final results. I don’t think we’ll have a Finals round. (People will already be too exhausted to even think about gearing up for more stress.)
One other thing I thought about doing to keep people from dropping out was some kind of periodic “door prize” or “mini-tournament” with start and end times throughout the day/night. These would not be for points, just for fun and maybe some cash/prizes.
Anything else I have not considered?
This appears to be the easiest way to do all this.