Thank you very much, now I know at least how to get from a grob to the corresponding music object.
In order to "catch" the notes under a slur that aren't slur attach points ("middle notes") I tried to use a closure as the callback function and store information on the "slurred state" (in slur / not in slur) in a variable. Unfortunately the callback isn't executed for the notes in input order, but in a quasi-random fashion, so this technique isn't usable at all. It seems that a property callback function isn't a tool suitable for detecting and modifying all slurred notes.
I hope that sooner or later I will find The Right Tool for the Job (TM).
Jakub