OK, so this seems to be a 24 hour fast.
Salt (sodium) and glycerol, after drinking water, prolong the time in which the water is excreted from the body, but they both work only for few hours.
The idea of hyperhydration to delay dehydration sounds interesting for marathon runners (in order to avoid the need to drink during the race), but according to American College of Sports Medicine, only few hours after hyperhydration, you will likely lose the excessive water.
Attempting to hyperhydrate with fluids that expand of the extra- and
intracellular spaces (e.g., water and glycerol solutions) will greatly
increase the risk of having to void during competition...
Glycerol can also have side effects, such as diarrhea.
I am not aware of any nutrient, supplement or food that would keep the water in your body in any meaningful manner in a 24-hour fasting scenario.
The effect of fasting is that you feel the lack of benefit of food. The resulting hunger can then remind you of things, other than food, that are important for you. Resisting from food alone (while drinking water) can already have this effect. Hyperhydration before fasting would cancel the effect of resisting from water, so what's the point of this, anyway.
Saying that, I am not promoting or suggesting anyone to resist from drinking water for any amount of time, because it could be potentially health- or even life-threatening.
If one has to survive for 24 hours without water and food, the means to avoid dehydration can be:
- Keep yourself well hydrated before fasting.
- Avoid alcohol and caffeine before fasting because they can promote the excretion of water through the urine.
- Avoid/limit anything what promotes sweating, such as exercise, exposure to sun and excessive clothing.