:( No such luck But then, looking further into config files, I found a way - just copy what the Ditto snippet code (the stuff in Manage Resoruces, that is) does when presented with OrderBy:
$orderBy = array('parsed'=>array(),'custom'=>array(),'unparsed'=>$orderBy);
I have only a vague idea what all those arrays are about, something to do with Ditto handling of custom sorts and filters, but I’m not sure I want to spend a week unpicking the code.

Main thing is, it works, and should work reliably!
KP