I over torqued my seatpost, used massive rear offset, and was blasting away with my glutes at about 40kmh when crack, I broke my seatpost and an expensive Ergopost too (but I had already damaged it when I was still into forward offset). Until the new (cheap Chinese) post arrives I have pushed three pieces of household rail steel tube up inside the post, one inside the other. I hope that they don't bend, and that I don't crack my frame. It only has to last out to the end of today.
Cracked Seatpost Repair Attempt by
Timothy Takemoto, on Flickr
On the other hand, if I had some strong carbon or steel tubing that was just the right diameter (2cm?) then it could perhaps be a permanent fix!