Hmmm....
You know how the cross the strings over to get more length than they would making them go straight? What if they made them go upwards too? That would allow for a greater length, but not such an increase in length of the piano. It would just be taller. But I can imagine how the weight would affect it.
What about making the piano out of something light? I mean, surely there are parts of the piano where it doesn't matter about materials, therefore they could be made out of a really light material, thus saving on the weight.
You say it wouldn't be possible, but are you sure it would be completely impossible to make? Disregarding money and transportation, would it be possible? Also I know that Fazioli do a 308cm piano- do you know of any bigger than that? I'd love to play it and hear what the bass sounds like

Henrah