Hi Manuela,
That works of course. I think I use \shape and \shapeII more than anyone in existence! I find these functions necessary and indispensable.
But I suppose the difficulty of asking questions is hat often we don't make out implicit assumptions explicit. What was rather hoping is that, what you would naturally expect when telling the tuplet bracket to avoid the slur is that it should actually, ah, avoid the slur, yet it manifestly does not.
So is this a bug or a defect or just bad behaviour in lilypond? What is really going here? I confess I have not done thorough searches of the bug list about this before posting, but perhaps someone who knows could illuminate?
Andrew