How to have everything noted in C major even if transposed for another instrument

[Alain]

Hi NG

In modal jazz it can make sense to write everything in C major key
because it is not always obvious what key a piece is in.
All the flats and sharps will then be noted at every appearance.

My problem is the following. I define a melody in concert key:

melody = {
  \key c \major
  \time 4/4
  c4 cis4 .....
}

then I use this melody for a concert sheet and a Bb sheet.
For Bb of course I use

\book{
  \score{
    \transpose bes c' <<
      \context ChordNames \chords { \override ChordName #'font-size = #6
          \harmonies
      }
      \context Voice = "mel" {
        \melody
      } 
    >>
  }
}

And here's the problem. The Bb sheet is now notated in D major. But I'd
like to have it in C major as well!

thanks for any help

Alain