I've since modded poster 8's script so that instead of using one predefined paragraph style a la
var source = 'GREPSourceStyle';
I've used Ariel's function for selecting a Paragraph Style that creates a UI with a dropdown to select the paragraph style with the desired GREP styles to copy across. His function can be found here:
http://forums.adobe.com/thread/681085
The script then runs as per normal - another UI appears to select one or many paragraph styles... and after clicking OK, the selected paragraph styles then have the GREP styles applied to them.