change format of bookOutputSuffix

[Kieren MacMillan]

Hi all, 

What’s the simplest way to modify the format of \bookOutputSuffix? For example, 
say I want the trailing hyphen to be an underscore — i.e., currently I get

  MyPiece-suffix.pdf

and I want

  MyPiece_suffix.pdf

Do I have to [locate and] rebuild the music function, or is there an easy 
parameter I don’t know about?

Thanks!
Kieren.