How to write a ly:pitch? as element of a pair?

[Urs Liska]

Hi all,

I have a with block where I would like to let the user enter a pair as a 
value, and one of the elements of the pair should be a pitch.

Something like:

\with {
  transposition = #'( d' . "D")
}

But this doesn't work, just like

  transposition = #'( #{ d' #} . "D")

What I currently do is have them write

  transposition = d'
  transposition-label = D

and cons them together within the function. This works but is inconsistent.

So is there a convenient way to write a pitch within a Scheme expression?

TIA
Urs