Dear David, I tried it, as You've suggested, with \slashed-char ##\D, but it doesn't work. I think I will try to use postscript-code in my library for function symbols. Are there other people interested in a library with functional symbols?
Is this something, that could be of interest for the 2.18. version of lilypond? As far as I know the functional symbols are not only used in germany and as far as I know they are neither implemented in sibelius nor in finale.