Tempo changes as metronome mark events

Hello everyone,

I'm currently working on a quartet and want to know your opinion about 
the tempo marks that may have a dashed line (i.e. **accel.**). My 
dilemma is as follows (take into account that I will need to output not 
only the score but also the parts): I could use the text span engraver 
to print such a line, but I will have to do so for the four instruments, 
given that parts must have the tempo indication; but this would print 
four times the same line in the score, where only the upmost line is 
needed. In the inverse sense, I could only use the line in the upmost 
instrument, but parts will not get the tempo change. So it seems the 
text span is not the best option.

The other option I'm thinking about is to program my text editor to 
comment and uncomment lines from the parts depending if I'm printing the 
parts or including them in the score. But it would create an unnatural 
notation, not to say is not really a "lilypond" solution.

The ideal solution would be the mix between the metronome mark engraver 
and the text span engraver. But I don't have the scheme nor lilypond 
skills to program something like that. What are your approaches to this 
kind of problem?

Best,

Juan