I am not connected to the world of modern notation, but I
cannot envision any musical meaning for the exact vertical position of
an OttavaBracket.
This is not true. There are many and many cases in which you need to tune the position of the ottava bracket as well as any other bracket.
And I'm not talking about modern notation. Even in nineteenth century notation this is absolutely necessary.
The first obvious example is when you have slurs near brackets. Which happen *very frequently*
In this cases a common algo is to:
1) choose which of the two objects has to be placed above (there's not a rule for that: it depends on aesthetical choices)
2) move them according to decision 1) and then *tune* their coordinates (which is tedious even with WYSIWYG editors, and requires trial-and-error even with them!).
Please note that you can't simply say: "ok, let's move this up and this down and all is done". You have to make heavy micro-tuning as a consequence.
I really can't count how many times I had to to that in so many scores. And there's no way to do that automagically. Really no way.
Otherwise I would not have asked what I'm asking.
Then I'm forced to use the extra-offset property for now.
Hope that someone could solve this issue. I'm sure there's a way for doing that, without changing the code and whole Lilypond would have a great benefi from it.
Best,
Paolo