RehearsalMark alignment

[Kieren MacMillan]

[Lilypond 2.13.47]

Hi all,

I can't seem to get quite the right set of overrides to have RehearsalMark 
behave as follows:
 1. If middle-of-system, center on the barline.
 2. If beginning-of-system, center on a small empty column ("invisible bar 
line") following the prefatory material.

My guess is this will require a callback function, but I'm not certain…
Any help or links to snippet(s) would be appreciated.

Thanks,
Kieren.