Call A Registered Dietician!
Find a Registered Dietician (RD) around your area. You're looking not just for a Sports Nutrition person, but a person that has the RD title, but you might find this type of person doing sports nutrition. What you're looking for here is a person that can give you the straight talk on nutrition, not the magic.
You already know about water and electrolyte replenishment on long rides. But, how to stack the snacking throughout the day to keep everyone and peak levels and from bonking? This is the question that you are trying to answer. The question after that is what kind of "fun" stuff can you put out to make the ride memorable that fits the nutrition equation?
What you want is a progression: slow burn fuels in the morning like bagels, muffins, and the like will help to fuel up for the remainder of the day. At the end of the day, bring out the candy, chips, and soft drinks. Complex carbohydrates to simple carbohydrates. Don't have a clue what you would do for lunch -- Some sort of veggie wrap thing in a tortilla w/mayo (carbs, texture, fat, easy to digest). Maybe a hunk or two of sandwich meat.
An RD will help you to select the foods to keep the fires stoked in everyone. Maybe go to a local hospital and beg for some pro-bono work for the community for your ride. Take it from a scientific standpoint, then find all the goodies that fit into that. Next, get the supermarkets to kick in a few cases of Fig Newtons, a few dozen bagels, and put up a sign: provided through the generosity of ABC Grocery. Or, maybe they give you a big 'ol discount (especially if you are non profit).
Just some thoughts