\textLengthOn in polyphony and over MultiMeasureRests

[Urs Liska]

Hi,

as a follow-up to my previous thread about text formatting 
(http://lists.gnu.org/archive/html/lilypond-user/2018-07/msg00395.html) 
(of which I'll soon post the results, I hope) I ran into some problems 
handling \textLengthOn.

The following snippet shows two things: \textLengthOn is a global thing 
(markup in voice one will push the music in voice two), and it doesn't 
work over MultiMeasureRests:

%%%%%%%%%%%%%
\version "2.19.82"

{
  \displayMusic \textLengthOn
  \override TextScript.self-alignment-X = #CENTER
  \override TextScript.staff-padding = #3

  c''1 ^\markup "Some longer text"
  \hideNotes
  c''1 ^\markup "Over an 'empty' measure."
  \unHideNotes
  <<
    {
      \hideNotes
      c''1 ^\markup "Other voice interferes."
      \unHideNotes
    }
    \new Voice {
      c'4 c'2.
    }
  >>
  \break
  c''1
  R1 ^\markup "It doesn't have any impact on MultiMeasureRests."
  c''
}
%%%%%%%%%%%%%

So I have two questions:

a)
I can see that it is considered too complex to automatically calculate 
\textLengthOn for one voice and have the other voices fill in the 
resulting width. But is there some way to emulate the result? What I 
need is the text filling a certain amount of time (typically a full 
measure) and stretch that amount of time to the text's width while 
another voice has some content in that measure..

b)
Is it possible to apply \textLengthOn over a MultiMeasureRest without 
resorting to a hidden extra voice?

Thanks
Urs