Convert PS/PDF to grayscale (vector)

[Alberto Mardegan]

Hi all!
  I'm looking for a way to convert a PDF file to grayscale, without 
compromising the quality (and possibly the size of the file) too much. I 
found the gs driver psgray that does a good job, but unfortunately it 
also rasterizes the file.
Somewhere I found the utility pscol which converts all vector graphics 
of a PS file into grayscale, without other losses of information, but 
unfortunately it leaves the bitmap objects untouched (i.e., they remain 
in colors).
Is there some parameter to pswrite (or pdfwrite) that does the trick?
Or any other ways for me to get to the goal?

I was also having a look at GS sourcecode, just in case I'll ever find 
the motivation to do the hack myself; what should be done there, to add 
a driver option (to pswrite) to convert everything into grayscale? I 
found the functions psw_copy_color(), psw_fill_{path,mask,rectangle}() 
and psw_stroke_path(); is that all? How about fonts?

Sorry for asking such many questions all at once. :-)
TIA,
  Alberto