TimeSig staff in sync with PianoStaff

[Kieren Richard MacMillan]

[ Lilypond 2.8.1 ]

Hello, all!

I'm writing and engraving a piece for small ensemble. I'm using  
"modern time signatures" (i.e., on a separate line above the system),  
and the conductor would like a second TimeSig staff just above the  
piano staff (the bottom-most staff) as well.

No problem... until the piano is tacet for some period of time  
(French score), at which point we'd like the TimeSig staff to  
disappear as well.

In other words, the second TimeSig staff needs to be synchronized to  
appear and disappear precisely when the PianoStaff does, as if they  
were one staff.

What's the easy way to do this?

Thanks,
Kieren.