Look at the funn seat/post combo with 8mm cromo rails. A pain to get assembled and adjusted, but one of the strongest designs ive seen, and it should never slip or snap. I do realize you were looking for a seat only...but this is one durable setup.
I've had good luck with Selle italia saddles. I'm currently running a Selle Italia Oktavia and its held up excellently so far. It seems that some great deals can be had on S.I. saddles on the net. The Flite ones are really sweet looking and have great reviews as well.
Didn't Thick or BMW or someone make an integrated saddle/cromo post that weighed like a kilo or something? That'd be right up your alley...ridiculously heavy nd expensive but unbreakable and unique.
(Oh, and to be realistic, either just get cheap, cheap BMX seats and throw them away when they bend, or get roadie saddles with Ti rails and throw them away when they bend slightly less often. I use Performance-brand roadies and they're cheap and strong and light.)