Man, you are a man. ) Kudos to your tenacity!
I'm not sure if I agree with that to be honest. making a player waste his time and his resources on something that cannot be done seems a lot worse to me than just making the area unreachable.
Well, I should agree here with you.
It would be better if that would be very HARD, but passable area, like you have 1% to succeed.
It doesn't mean, that it should be done at all cost, just "it would be nice if".
After all, all that need resource to do this.
Also - I remember Blades of Avernum, first scenario from three. There was dragon in first part, that shouldn't be killed, but I actually managed to do it, and that was very satisfaying, like posting pics of it on forum. )))