rehearsal marks: separate from content encoding and display above strings

[Thomas Weber]

There are two things about rehearsal marks in orchestral scores I'd like to 
know about.

Firstly, how can I separate rehearsal marks (and other structural stuff like 
\break or \pageBreak) from the actual musical content?  Can I somehow add a 
virtual staff to carry the rehearsal mark information?  I think I remember 
seeing something like that before, but I can't seem to find any documentation 
about it.

Secondly, how can I make rehearsal marks appear both on the very top staff and 
the top staff of the strings?  As far as I can see, rehearsal marks will always 
be put at the very top staff.

Thanks!
Thomas