a "how to" question: "floating" time sigs

[Kieren MacMillan]

Hello, all --

This is just a "what would be involved" question, to see the scope of  
this concept, either from a "cool project to build towards as a  
develop myself" or "cost of sponsored feature"... In other words, if  
someone could explain what would be involved (fairly technically),  
plus a quick estimate on the sponsor cost, I'd appreciate it. [Heck,  
there might even be a way already that I don't know about!]

I often need to use "modern time signatures", i.e., with them  
"floating" above the system (top staff, and sometimes piano staff as  
well) and not contained within the staves themselves. Although the  
tip/trick in the docs (with the \TimeSig is okay, what I've taken to  
lately is to defining the macros

staffMakeTS =
{
        \override Staff.TimeSignature #'font-size = #2
        \override Staff.TimeSignature #'extra-offset = #'( 0 . 7 )
        \revert Staff.TimeSignature #'stencil
}
staffMakeNonTS =
{
        \revert Staff.TimeSignature #'font-size
        \revert Staff.TimeSignature #'extra-offset
        \override Staff.TimeSignature #'stencil = ##f
}

and then calling these in the uppermost staff of the score and upper  
staff of the PianoStaff, whenever it is appropriate for the score.

This works like a charm, with the added bonuses that the time sig is  
"synchronized" with the PianoStaff in Frenched scores (i.e., it  
appears and disappears with the PianoStaff itself, because it's  
*part* of the PianoStaff!), and the elimination of the whole TimeSig  
context, with its extra set of skipped bars, etc.

The irritation (tradeoff) is that in every Frenched score, each  
system rebreaking (i.e., for different editions, after space or page  
tweaks, etc.) may require a resetting of the \staffMake(Non)TS macros  
in order to ensure that the correct "uppermost staff" is the one  
that's contributing the time sig.

What would be great is to allow floating time signatures to be added  
directly to certain staves the way RehearsalMarks are -- i.e., if I  
want the time signature to be above the uppermost staff in the score  
(system) plus above the PianoStaff, then I could add a  
floating_time_signature_engraver to those contexts. (This engraver  
should be separate from the mark_engraver, of course, because one  
might want the PianoStaff to use one or the other but not both,  
whereas the uppermost staff would likely always have both.)

Any thoughts, comments, suggestions, estimates?

Best,
Kieren.