Positioning volta bracket

[Gregory Heytings]

Hi list,

Is there a way to position (or to adjust the position of) the volta 
brackets manually?  With the following example:

\relative c'' {
  \repeat volta 3 { d4 d d d }
  \alternative { { c4 d e f } { e4 f g a } }
}

the volta brackets are raised, because of the final "a" note, but it could 
have fit under the volta bracket at the "normal" position, since it does 
not clash with the "3." indication.

I tried

\layout { \context { \Staff \override VoltaBracketSpanner #'Y-offset = #N } }

with various values for N, without success.

Thanks,

Gregory