A bit late, but better than never:
I have to admit that I am rather a live musician, so I haven't even heard of that tool and an FFT filter before your post
Thanks for your input, but I'm afraid that solution will be out of question for me, after checking the possibilities:
My problem was not really to get the (non)existing piano out of that song.
It's rather the vocals (which are not difficult, and are performed by the right piano hand) and a complete, somewhat mixed together orchestra, which acts as bass and chords (that has to be recomposed into a working left piano hand). With the active orchestra instruments changing all the time -as far as I understood the FFT possibilities- it would not really be possible to identify those overtones correctly. And an FFT filter regarding each single orchestra instrument...well...a bit too much, I'm afraid...)
Thanks again for your input, but I'm afraid it will result in some patient "listening and playing along"...