\paper {
  ragged-right = ##f
  indent = 2.5\cm
  tagline = ##f
}

% Solution 1: Using a simple markup with a particular halign value
% Drawback: It's a markup, not a dynamic command, so \dynamicDown
%           etc. will have no effect
semppMarkup = \markup { \halign #1.4 \italic "sempre" \dynamic "pp" }

% Solution 2: Using a dynamic script & shifting with
%             \once \override ...X-offset = ..
% Drawback: \once \override needed for every invocation
semppK =
#(make-dynamic-script
  (markup #:line
          (#:normal-text
           #:italic "sempre"
           #:dynamic "pp")))

% Solution 3: Padding the dynamic script so the center-alignment
%             puts it at the correct position
% Drawback: the padding really reserves the space, nothing else can be there
semppT =
#(make-dynamic-script
  (markup #:line
          (#:normal-text
           #:italic "sempre"
           #:dynamic "pp"
           #:hspace 7.1)))

% Solution 4: Dynamic, setting the dimensions of the additional text to 0
% Drawback: To lilypond "sempre" has no extent, so it might put
%           other stuff there => collisions
% Drawback: Also, there seems to be some spacing, so it's not exactly the
%           same alignment as without the additional text
semppM =
#(make-dynamic-script
  (markup #:line
          (#:with-dimensions '(0 . 0) '(0 . 0)
                             #:right-align
                             #:normal-text
                             #:italic "sempre"
                             #:dynamic "pp")))

% Solution 5: Dynamic with explicit shifting inside the scheme function
semppG =
#(make-dynamic-script
  (markup #:hspace 0
          #:translate '(-18.85 . 0)
          #:line (#:normal-text
                  #:italic "sempre"
                  #:dynamic "pp")))

% Solution 6: Dynamic with explicit alignment. This has only effect
%             if one sets X-offset!
% Drawback: One needs to set DynamicText.X-offset!
% Drawback: Aligned at the right edge of the additional text,
%           not at the center of pp
semppMII =
#(make-dynamic-script
  (markup #:line (#:right-align
                  #:normal-text
                  #:italic "sempre"
                  #:dynamic "pp")))

\new StaffGroup <<
  \new Staff = "s" \with { instrumentName = \markup \column { Normal } }
  <<
    \relative c'' {
      \key es \major
      c4\pp c\p c c | c\ff c c\pp c
    }
  >>
  \new Staff = "sMarkup" \with {
    instrumentName = \markup \column { Normal markup }
  }
  <<
    \relative c'' {
      \key es \major
      c4-\semppMarkup c\p c c | c\ff c c-\semppMarkup c
    }
  >>
  \new Staff = "sK" \with {
    instrumentName = \markup \column { Explicit shifting }
  }
  <<
    \relative c'' {
      \key es \major
      \once \override DynamicText.X-offset = #-9.2
      c4\semppK c\p c c
      c4\ff c
      \once \override DynamicText.X-offset = #-9.2
      c4\semppK c
    }
  >>
  \new Staff = "sT" \with {
    instrumentName = \markup \column { Right padding }
  }
  <<
    \relative c'' {
      \key es \major
      c4\semppT c\p c c | c\ff c c\semppT c
    }
  >>
  \new Staff = "sM" \with {
    instrumentName = \markup \column { Set dimension "to zero" }
  }
  <<
    \relative c'' {
      \key es \major
      c4\semppM c\p c c | c\ff c c\semppM c
    }
  >>
  \new Staff = "sG" \with {
    instrumentName = \markup \column { Shift inside dynamics}
  }
  <<
    \relative c'' {
      \key es \major
      c4\semppG c\p c c | c\ff c c\semppG c
    }
  >>
  \new Staff = "sMII" \with {
    instrumentName = \markup \column { Alignment inside dynamics }
  }
  <<
    \relative c'' {
      \key es \major
      % Setting to ##f (false) gives the same result
      \override DynamicText.X-offset = #0
      c4\semppMII c\p c c | c\ff c c\semppMII c
    }
  >>
>>

\layout { \override Staff.InstrumentName.self-alignment-X = #LEFT }