Vertical position of a breathe sign

[Jakub Pavlík]

Hi,

I'm using sort of non-standard notation - in four-lines-staff in the way in 
which gregorian chant is notated.
In this way I set up the staff:

\override Staff.StaffSymbol #'line-positions = #'(-7 -5 -3 -1)

So, the notes B, D, F, A lie on the lines.

But in this setup, the breathe sign is typesetted flying over the staff. I want 
it to lie on the top-most line, on the line A, but I haven't found any way to 
modify vertical positioning of this sign. Is there any?

Thanks,
Jakub Pavlík