Original Gonville actually has its own curly brace
Upon looking at his preview specimen represented here:
I noticed that there is a distinct brace!
The tar.gz/zip link actually has an .otf with multiple sizes for the brace. MuseScore could easily throw in one of these into the E000 position like Bravura for Gonville...
To get it to work, it would require to remove some of those lines in C++ that check "if Emmentaler or Gonville" having to do with the brace and just check for Emmentaler, and obviously also to update the .otf file with an included glyph. I think it should be done since it'd be fairly easy.