Can I make a TextScript occupy no space, potentially colliding?

[Richard Shann]

In this code I put a large letter A above the music.
If I uncomment the tweak the letter A is moved elsewhere, but it still
takes up space at the top of the score.
If I uncomment the override, I don't get the space back.

\version "2.18.0"
\score { 
 {
  %\override  TextScript.Y-extent = #'(0 . 0) 
  a1 
  %-\tweak #'extra-offset #'(7 . -12) 
  ^\markup {\fontsize #12 A }
  }
} 

Is there a way to stop LilyPond trying to prevent my TextScript
colliding - to say that it occupies, say, no space?

Richard