You might want to take a look at the book "Thrive" by Brendan Brazier. It is about a vegan diet for athletes. Lots of discussion about keeping nourished while in a long race:http://www.brendanbrazier.com/book/index.html
I agree that Brazier is one of the best authorities around on the subject. You might also want to check out Ruth Heidrich's website http://www.ruthheidrich.com
. Ruth is a ironwoman triathlete. She follows a mostly raw foods diet. In response to an email question similar to yours, Ruth told me that fresh fruit provides her with all the nutrition she needs while riding.
I haven't done 12 hour events yet, but I'll share some experiences doing bike rides of up to 8 hours. First, if you eat a high nutrient dense whole foods plant based diet your needs on a endurance event are probably not the same as the typical athlete. I no longer take any processed energy foods or drinks. I eat only whole plant foods off the bike and see no reason to change that on the bike. I mix my own drink, which I carry in three water bottles on the bike. What works for me is organic apple juice mixed 50/50 with water. I also carry a raw trail mix and bananas on the bike. The raw trail mix has a mix of nuts, seeds, dried fruit and plain uncooked oatmeal. Ruth Heidrich, as I understand it carries lots of bananas taped to her bike frame. Brazier mixes up his own energy drink, but I am not sure what it is. He advises eating and drinking small amounts periodically during long events before hunger and thirst strike.
I have done lots of events and training rides in very hot weather. I live in the SE. I have never cramped up on the regimen I described above. I add no salt to any foods in my daily routine. I take no electrolyte supplements on the bike. As I understand it, if you don't consume lots of salt your body gets good at retaining it, and you don't lose as much during exercise. My experience is that low salt intake off the bike means I don't need electrolytes on the bike. Sounds counterintuitive, but that has worked for me.
Bottom line: eat whole natural plant foods both on and off the bike. Make your own whole foods energy drink(juice and water, for ex.) to carry with you. Keep salt low off the bike.