\version "2.19.47"

% Create a "watermark" style time signature with the following characteristics:
% - Remove time signature from all contexts where it is not explicitly included
% - font-size settable
% - top-align to the given staff
% - print behind music, without affecting any layout of music
% - flow through following stave without pushing them down

% By default time signatures are not printed at the Staff context
\layout {
  \context {
    \Staff
    \remove "Time_signature_engraver"
  }
}

% Add a watermark time signature to a given staff by writing
% \new Staff \with {
%   \watermarkTimesig <color> <font-size>
% }
% (with the color argument being optional, defaulting to an 20% grey)
watermarkTimesig =
#(define-scheme-function (color font-size)((color? (list .8 .8 .8)) number?)
   #{
     \with {
       \consists Time_signature_engraver
       \override TimeSignature.font-size = #font-size
       \override TimeSignature.X-extent = #'(0 . 0)
       \override TimeSignature.space-alist.first-note = #'(fixed-space . 0)
       \override TimeSignature.color = #color
       \override TimeSignature.layer = -1
       \override TimeSignature.Y-offset =
       #(-
         2 ; 2 staff spaces because it’s normally center-aligned
         (* 2 (magstep font-size))) % two staff spaces = one digit height
       \override TimeSignature.Y-extent = #'(+inf.0 . -inf.0)
     }
   #})