I just found a Katoomba to Mittagong book (actually a series of photocopied pages from a book) that someone has obviously given me years ago and I've completely forgotten about? Anyway, in it has the suggested route of once crossing the Cox heading up and over Wonga Mt via Oak terrace and then connecting with the Kowmung which you follow upstream upon exiting via one of the Spurs that lead to Mount Field.
In all the trip reports and general conversations on the matter I've never heard this route mentioned nor took...wonder why it dropped out of favour as the preferred path?