Mixing music and markup in Scheme

[Phil Holmes]

Yet another question about Scheme functions (note - I _do_ try to find the 
answers to my questions by reading the manual and experimenting, and have 
been looking at this for a couple of hours without success).

Background: Renaissance music has a somewhat cavalier attitude to 
placement of accidentals.  Sometimes they are conventionally placed to the 
left of the note, and occasionally they are placed above the note, like 
musica ficta in modern transcriptions.  If I use \set suggestAccidentals = 
##t in Mensural music, I get a modern accidental sign, so this doesn't 
help setting these.  A long-hand way of getting this effect is:

mus = \relative c''
{ c4 d e 
  \tag #'mod { fis } 
  \tag #'mens {\once \omit Accidental fis 
    ^\markup { \musicglyph #"accidentals.mensural1" } } 
}

\score {
\keepWithTag #'mens {
    \new MensuralStaff
    {
      \new MensuralVoice { \mus }
    }
  }
}

\score {
\keepWithTag #'mod {
    \new Staff
    {
      \new Voice { \mus }
    }
  }
}

I've tried to write a function that replaces all the tag ... stuff, so 
that all I need in the music is

\MensFict fis4

but I can't see how to return the music and the markup in the same 
function.

Guidance please?