change note heads according to their current measure position?

[Marc Hohl]

Hi list,

I want to change the note heads according to their current position 
within the measure. More precisely, I want eighth notes that are on a 
beat to keep their normal appearance, the notes off beat should be changed.

In ASCII pseudo-notation:
 _           _
| | |   |   | | | 7 |\ 7 |\
o o o   o   o o |   o    o

should be changed automatically to
 _           _
| | |   |   | | | 7 |\ 7 |\
o x o   o   o x |   x    x

(the '7' here stays for a r8)

IIRC, there is no way to obtain the current measure position from the 
NoteHead grob itself ...

Any ideas are highly appreciated!

Cheers,

Marc