Set midiInstrument for chord mode

[Anders Nelstrom]

Is there a way of setting the midiInstrument in the context of a  
chordmode block? By default, the chords are played by a piano in the  
midi output, and I would prefer to set it to a drawbar organ. The  
following is an abridged template to show what I am using:

melody \new Voice { \relative c' {
        ...my tune...
}}

harmonies = \chordmode {
        \set Staff.midiInstrument = "drawbar organ"
        ...my chords...
}}

\score {
        {
        <<
                \new ChordNames {
                        \set chordChanges = ##t
                        { \harmonies}
                }
        \new Staff \melody
        >>                
        }
        \header{}
        \layout{}
        \midi{}
}

I have tried using the command:
        \set Staff.midiInstrument = "drawbar organ"
within the \chordmode{} context, but it has no effect. I suppose this  
is because it doesn't use a Staff? I also unsuccessfully tried:
        \set Voice.midiInstrument = "drawbar organ"
which I doubt is even a legal command (searched the docs for it, and  
no hits).

Any suggestions?

Thanks