Fernandito:
haha!
I've done a similar thing, but nowhere near as crazy! It was...hmmmm...let me think. I believe it was during a Chopin nocturne. Yes, that's right, op. 55 no. 1, a piece that I was never entirely sympathetic to, and which I wasn't very interested in learning at first, but it was actually the first piece my last teacher assigned me and I wanted to give him a good impression. At the end there is that descending deal with both hands playing two separate melodies (though it is by no means complicated counterpoint or anything) with rather awkward fingerings, and that section is followed by a long arpeggio sequence that I never really took the time to *learn*. Well, as might well be overwhelmingly obvious I more or less made both things up as I went, but at the same time the end result was pretty similar to what was actually written...
The most I've "screwed around" was in the HR6 when I performed it at my school to musically illiterate people. I skipped that C-sharp major transition section because I couldn't play it, I changed all of the cadenzas at will and on the spot, wrote in at least two of my own (short) cadenzas, and changed the dynamics moderately here or there. It was partially modeled after one of Cziffra's recordings...not quite as successful though!
But to put it more generally for the topic, I advocate changing scores around on the spot for exciting spontaniety, but it depends on the music. I have no problem messing around with the HR6 for instance, but would never touch a note of a Chopin Ballade, or indeed a "better" Liszt piece. And as for playing unique or obscure pieces, I am also quite interested in it but haven't had much chance to play such a piece lately, and probably won't for a while unless I can whip up a few Godowsky etudes or Alkan pieces. I think the best I can do is Ornstein's "9 Miniatures" which I hope to learn and perform over the summer, or perhaps Alkan's 3 Romantic Andantes.
One thing that has more appeal, though, is unknown Baroque, pre-Baroque, or Classical music--and for the latter I consider Haydn to be unknown, though there are hundreds of better examples. I'd like to play pieces like Gould used to play, from Gibbons and Byrd and so on, and even some pieces by earlier salon pianists like Hummel, Kalkbrenner, and Moscheles, whose works may not be upper-class but are certainly worth it for creating that kind of historical atmosphere!
Topic: Going against the norm