The wire is only part of the problem. Sustain is usually pretty short in the last octave, making it not so useful.
More importantly, C8 is 4.2kHz, so the second harmonic is 8.4kHz and the forth one is at 16.8kHz which is one many people can not hear already, and those who can hear often have a low sensitivity and and a disliking sensation about it. All these numbers to say that if you were to extend of another octave to C9, that last note (assuming you could make the instrument sound it "perfectly") would not be pleasant to most people: too "dull" for the majority, who will hear only the fundamental at 8.4kHz and unpleasant for the majority of those who would would hear the second harmonic. If you go to the piano technician forums, you already see a number of people complaining about current established last octave….
If you are instead asking about a couple of semitones, probably it can be done, but is it worth it? I doubt it. Piano is such a conservative industry….
