Ian, not sure of your tri experiences but if you've done Iron distance then you might have some nutrition experience at that level / distance which can be somewhat translated across to this run (given that the event times may be similar in duration).
Obviously there are tons of products on the market but I have had very good experiences using a combination of Endura Optimizer (powedered supplement, mixed with water), gels and Saltstick electrolyte tabs for endurance events up to 14hrs (I guess you've probably heard of Opti and Saltstick), and I am aware of many others doing the same. That said, I think a lot of people would 'top up' their nutrition in the back half with a sandwich or two, bananas etc.
In an Ironman, for example, you should typically consume roughly 1 gram of carbohydrate per kilogram body weight per hour of exercise.
In a longer enduro event, you can probably get away with slightly less (eg somewhere between 0.75-1 gram of carbohydrate per kilogram body weight per hour) but you need to have a rough idea of your potential timeframe: clearly there is a big difference in carb requirements if you are thinking 12hrs but take 18hrs instead.
The types of foods and fluids which are most suitable depend on the intended intensity of your exercise and your individual tolerance - and I note your comments about holding down solids. The higher your intensity (and, again, probably linked to whether you are aiming for 12 or 18hrs!), then go for options which are easily digested and absorbed - sports drink, Opti, gels and lollies. If you are aiming for lower intensity, then unlike your experiences in triathlons / marathons you might be able to tolerate foods such as fruit (esp. bananas), cereal bars, sports bars, plain sandwiches, crackers, yoghurt, nuts etc. It is useful to include a combination of sweet and savoury foods to avoid 'flavour fatigue'. Including some salty options such as Vegemite sandwiches, pretzels, crackers, potato chips etc. can also be useful, especially in very hot conditions or when fluid losses are high.
Sorry if I'm repeating stuff you already know but hope it is otherwise of some benefit. I guess you can always over cater and have some different food available in your special needs bags at different feed stations, even if you don't ultimately end up eating it...
Anyway, whatever you do, make sure you practice your nutrition before the event! And, yes, Monique Ryan's book is extremely good.
Cheers
