"Toner/Ink Saver" implementation?

[gary carroll]

It would seem that using CUPs and / or Ghostscript it would be pretty 
straightforward to do a “toner / ink saver” type of printing to printers 
that do not incorporate this feature in their firmware. Basically, rip 
each page at less than 100% saturation.
I don’t want to re-invent the wheel… however, it is not clear to me how 
to do this simply. Is there some parameter that I am missing?