Yeah, I would like to see sieges go on for years... If they can't get in then they can't get in, I think dwarves would like that idea, it means they have built an impervious fortress. My forts usually have respectable supplies. Even if farming was turned off it shouldn't even need rationing in order to outlast the current sieges. If your fort isn't actually under any pressure than it shouldn't really be under any pressure...
At this point in the game's development there are holes in the game, the impervious 1x1 channel is one of them. I don't think it is currently practical to start trying to balance sealing your fortress. If you don't like it then don't use it. I think that this is a perfectly reasonable attitude to take towards an unfinished game.
The idea of an invulnerable fortress is not all that bad. Maybe not in the literal sense, but I could easily imagine a siege not wishing to attack the dwarves on their own ground, and actually choosing to sit outside and fortify their position. It would be like an off switch on the outside world, no above-ground trading, lumber, water, fishing, hunting... Maybe with occasional attempts to break the siege coming from your allies, of course, by that time the siege is dug in and it becomes goblin fortress...
How bad do you want this to be, between sieges the negative impact would probably have dissipated, so should this be pretty much fatal to anyone who closes the door upon the words 'siege' and keeps it closed until the siege ends?
I could envisage a fortress being concerned about their reputation, which would be negatively impacted by traders never returning and envoys being fed to the fluffy wamblers... Unless they are isolationists, which is an option I would like...