Hi Urs,
No solution from me, but I just feel compelled to mention that this is one of my BIG issues with lilypond that I cannot seem to control - adjusting fine spacing like this in my scores is never successful. If there is a way to do it, hopefully somebody will let us know. If not, this would be a hugely desirable enhancement. I also have issues with grace note spacing becoming uneven when used in the context of cross staff beaming, and compounding that with the use of dodecaphonic accidentals makes lilypond produce very ugly output. It's about my only criticism of the entire marvellous application.
Andrew