accessing the text stencil of a TextSpanner

[Urs Liska]

It is possible to access/reshape a Metronome Mark like this:

\override MetronomeMark #'stencil =
    #(lambda (grob)
       (ly:stencil-scale (lyric-text::print grob) 0.78 1))

(which will squeeze the font to a width of 0.78 (which surprisingly 
matches the font of the printed model very well).

Now I need to make TextSpanners identical to the MetronomeMark, so I 
would need to apply the same scaling to
    TextSpanner.bound-details.left.text

but I have no idea how to achieve that.

\once \override TextSpanner.bound-details.stencil =
     #(lambda (grob)
       (ly:stencil-scale (lyric-text::print grob) 0.78 1))

doesn't have any effect.

Any help?

Urs