best way to code music with instrument-switch

[Kieren MacMillan]

[Lilypond 2.11.49]

Hello all!

I need to write out some scores and parts that have multiple  
instrument switches (e.g., flute to alto flute to piccolo) per movement.

I like to keep my key signature stuff in a \global variable, but then  
each new key signature does not appear correctly (i.e., transposed)  
for the transposing instrument(s).  =\

So what I've been doing instead is splitting my music up into  
sections, with a new section any time there is an instrument switch  
-- as you can imagine, this makes for ugly Lily-code.

Is there an obvious way to handle this -- using a combination of  
\transposition and \transpose -- which would allow me to:
    1.  have a single \global variable;
    2.  enter each player's music, in concert pitch, in a single  
variable; and,
    3.  output a C score and a transposed score (and parts), with  
the correct key signatures in each Staff?

Thanks!
Kieren.