Oh jesus I just read the thread. Seriously, no one so far knows why beer doesn't dehydrate you per say, but the main reason you're hungover is because you're dehydrated?
Beer is mostly water. In this way it hydrates you.
One of the effects of alcohol is that it inhibits
ADH, or Anti-Diuretic Hormone (Diuresis btw is peeing). ADH is the hormone that controls water retention. Your kidneys main function is to eliminate wastes, but to concentrate them in urine by removing as much water as possible (from the urine, and back into your body) along the way because, well, water is good for you and is important for living. Your kidneys have such an important job that they can actually influence your blood pressure and heart rate to get the proper filtration, which is why ADH, also called vasopressin, can control your blood pressure as well.
Anyway. Alcohol inhibits ADH (through the renin pathway), which limits your body's ability to control water reuptake in the kidneys. The result is you lose a lot more water than you would normally. This is where the dehydration comes from. The fact that you are drinking alcohol that has a high water content exaggerates this effect and causes you to have to pee all the ****ing time. (This is also where 'breaking the seal' comes into play: the stretch receptors in your bladder are not as sensitive to your bladder filling initially, but once you evacuate your bladder and 'break the seal' as it begins to fill up again the receptors start firing sooner and more often.)
I would guess that the higher the water content and the lower the alcohol content the longer you could survive without dehydrating. Sugar content is also very important because sugar, as a solute, requires some water to evacuate from your body as a waste product, which makes you lose even more water. This is why wine hangovers are way worse than beer hangovers.
So your best bet would be low cal & low alcohol beers. And if you could supplement with water you'd be in even better shape. But you'd still only last a handful of weeks before malnutrition set in.
Edit:
alobar