Hi Dan. Im currently browsing the web from a linux box, so, i cant test it right now. But i believe that this can help:
In the GPO folder, there is a sub-folder called "banks/standard". Look inside that folder, you will found a file called "GPO.CHR.bank.xml". Open the file with a text editor, preferibly an xml editor. Look for a section like this one:
you need to change "off" to "on".
<Define name="$sustain_sw" value="off"/>
Save the file
I suggest you that before do this, you backup the original file, in case this dont works.
Hope this helps.