i think you can definitely keep the weight off after fasting if you're careful, i lost 29lbs on the mc last year, i ate very lightly for a few weeks afterwards (mainly fruit and veg, and a small amount of protein eventually) and continued to lose weight. however i took a holiday to menorca a few months later and the healthy eating went out of the window, i even started drinking alcohol which i don't normally do (holiday mentality and all that lol!). anyway i figured that if i had stuck to my plan then i wouldn't have gained back, so i think it's probably up to the individual in most cases. i accept with everything there are exceptions to the rule but i would say if you're very careful then you should manage not to gain.
a long fast can break the physical ties to eating that we have but the emotional ties take a bit more work and can so easily rear their ugly heads at any time, especially once we go back to eating. i always say "if i never had to eat ever again, my problems would be no more...i'd be cured," just like an alcoholic or drug addict abstaining from alcohol or drugs respectively. the problem is if we don't eat, then we'll die....my happiest state is a fasting one (3 days + lol!), when the thought of food is a distant memory and i reach that euphoric light feeling where i think i've cracked the food addiction, the problem is it's still there when we go back to food.
anyway i've digressed a bit but, in a nutshell...yes i do think it's possible not to gain :)