I find very little difference in exertion when using or not using a dynamo. So while pedal-power may be inefficient overall, I would be surprised if there was a significant increase in food intake if you were already pedaling. Some days I ride a bike with a generator hub, and some days I ride another bike. I have not noticed myself eating more on the dynamo days.

To the OP: I think that in general, and using the supplied suggestions in particular, the way to solve all your various voltage problems is to go fully USB. USB works for many phones. Tablets may draw more power than the hub puts out when in use, but they may charge as long as the tablet is off or sleeping when charging. My plan is to use an external battery pack that is charged via USB, charge that up over the course of the day, and use that to charge my tablet and phone.

Using dynamo-powered headlights eliminates the need to try and power battery-powered lights. The Luxos U does seem ideal for what you want, and it's the device I've been lusting after. There are also USB-powered battery chargers, so if you had one of those and a couple of rechargeable AAs or AAAs, you might be able to supply all of your energy needs from one hub. Except a laptop. If you had a very efficient laptop that you used very seldom, and way to recharge it via USB, then maybe. And your speedometer still creates a problem. I don't know if you can get a rechargeable battery of the size usually used in a cycle computer, and if you could, it seems like it would be a lot of effort and expense to be able to say that you're completely self-powered. Like Looigi's point, the effort you put into not having that one, non-rechargeable battery probably exceeds the benefits of converting it to pedal power. Better you should see if there's a rechargeable cycle computer, or one that runs on AAAs, or just use your phone as the speedometer.