\version "2.16"

#(let ((inner-grob-interpret-markup grob-interpret-markup))
   (set! grob-interpret-markup
         (lambda (grob mup)
           (let ((font-size-mode (ly:grob-property grob 'font-size-mode))
                 (font-size (ly:grob-property grob 'font-size)))
             
             ; to see, whether this is actually called: #:with-color red
             (set! mup (markup #:with-color red mup))
             
             (if (and (eq? 'absolute font-size-mode)(number? font-size))
                 (inner-grob-interpret-markup grob (markup #:abs-fontsize font-size mup))
                 (inner-grob-interpret-markup grob mup)
                 )
             ))
         ))

\layout {
  \context {
    \Score
    \override InstrumentName #'font-size = #30
    \override InstrumentName #'font-size-mode = #'absolute
    \override TextScript #'font-size = #30
    \override TextScript #'font-size-mode = #'absolute
  }
}

\new Staff \with { instrumentName = "Noise" }
\relative c' { e4^\markup "Test" e e e }