extenders in figured bass without a figure in front

[C.Flothow]

Hej!
in a piece which I've been transscribing there is repeatedly an extender 
line in the figured bass without a number in front. This means of 
course: hold the accord.
I've tried to indicate this with a markup, but that's of course not the 
same.

% --------------------------------------------------------------
\version "2.14.0"

{\key g \major \clef bass
    d d'_\markup{------} c'
}
% --------------------------------------------------------------

Is there a way for producing a "real" extender line here? I've not found 
how to do that in the docs. Have I missed something?
Chris