Modify cautionary time signature

[Kevin Barry]

Hi All,

I would like to be able to apply an override to a cautionary time
signature (the extra one printed at the end of a line) without
affecting the one printed at the beginning of the next line. (I want
to modify it, not get rid of it.)

I have tried using \alterBroken (which rightly throws an error because
a TimeSignature is not a spanner). I also tried \offset, but it just
takes the first value from the list and applies it to both grobs. I
also tried the example callback in the "Difficult tweaks" section of
Extending, but that produces the same result as alterBroken. I'm at a
loss...

Any help appreciated,
Kevin