keep-alive-together (or hara-kiri-together?) to kill a Marks context

[Kieren MacMillan]

Hello all,

Apologies in advance: I’m right in the middle of a huge engraving project (past 
due!), or I would try to whip up a minimal example for my question…

I use a ScoreMarks (some call it MarkLines) context, containing RehearsalMarks, 
MetronomeMarks, etc.

I have a frenched score, with three ScoreMarks contexts:
    #1 at top
    #2 above vocals/chorus (StaffGroup)
    #3 above strings (StaffGroup)

I want the top ScoreMarks line to always appear (regardless of what the top 
staff actually turns out to be). But I want the other two to disappear — 
despite the fact they [always] contain ‘music' — if all staves have been 
removed in the StaffGroup that the ScoreMarks context sits above (e.g., if no 
string staves remain, then there is no #3 ScoreMarks).

A minimal snippet showing how this can be done (using keep-alive-together, I 
would assume?) would be greatly appreciated.

Thanks,
Kieren.