midi (and ONLY! midi) transposing

[Bric]

For scores with multiple instruments of various tunings, how do you tell 
lilypond to to render midi  with "normalized" pitch (everything adjusted 
to concert pitch, appropriately) — WITHOUT transposing the visual, 
written notes.

E.g., a trumpet part is written a whole tone higher, and you want to 
keep it written that way, but, at the same time, you wanna hear your 
trumpet part sound in tune with its concurrent piano part.

The way things are right now, one needs to create an additional 
lilypond, containing WRITTEN transposition, to get a score like that  
sounding right.  Or /is/ there are way to do it with a couple of code 
directives?

thanks