Seems that if I simply rename the "Quad7890-9890-K7" folder to "Quad9890-K7", it solves the problem! Not sure how this could have gone unnoticed for a printer that's been out for 5 years.
This is not listed in the installation instructions at http://www.piezography.com/PiezoPress/technical-support/epson-7890-9890-installation/!