I think it would be tough for me to get all my protein in for an extended period of time with only 1000 calories -- and that is always my focus, even when trying to drop a few pounds -- but it can be done if you choose your foods wisely. No matter how far out you are, you want to be sure you lose fat not muscle mass. Both times that I tried physician-supervised diets, they called for 1200 calories, but I don't know if a lower number would be a problem in terms of metabolism or not. It really depends on what your resting metabolic rate is and how long you anticipate being on so few calories. The body is able to rebound after short periods of restriction, but I have never read anything specific about how many calories for how long results in permanent lowering... just that it sometimes happens as a result of the extreme caloric restriction during the first months after WLS.
At 5 years out, I don't eat too much more than the 1200 calories normally just to maintain my weight with my messed up metabolism. Personally, I would not go any lower than 1000, but that is just my opinion based on anecdotal "evidence"... knowing that many times people have trouble losing at only 800-900 calories but start losing again if they increase the calories a bit.
10 years out; 190 pounds lost, 165 pound loss maintained
You don't drown by falling in the water. You drown by staying there.