adjusting a system's NonMusicalPaperColumn Y-offset by a relative distance

[Kieren MacMillan]

Hi all,

I often resort to explicitly moving systems, using the function

staffAdjust =
        #(define-music-function (parser location adj) (pair?)
                #{
                        \overrideProperty #"Score.NonMusicalPaperColumn"
                                #'line-break-system-details $adj
                #})

For 'Y-offset adjustments, I'd like to be able to give a relative  
adjustment (e.g., -10 means raise 10 spaces from where Lilypond would  
*normally* put it).
Is there a way I can get a callback function (giving the "current"  
position) in order to accomplish this?

Thanks,
Kieren.