removing time signatures engravers locally

[Paul Scott]

2.25.2:

I understand polymetric notation (NR 1,2,3).  I have a piece which has a 
polymetric section in the middle where the rest of the piece is not 
polymetric.  Some parts have the same time signature for the whole piece.

At the point where the polymetric section begins and ends I want to 
remove the time signature from the parts whose time signature doesn't 
change.

TIA for any help with this including Lilypond manual references if relevant.

Paul