Currently it is not possible to use a custom configured java.awt.print.PrinterJob when printing via FOP. This makes it somewhat awkward to use for printing via its API. See the implementation of org.apache.fop.render.print.PrintRenderer, specifically how instance variable printerJob is initialized in method initializePrinterJob().
There should be some way to parameterize printing w/ custom PrinterJob object. The most natural place for this (from the point of view of the caller) would be a method in org.apache.fop.apps.Fop, since the basic interaction w/ FOP happens via an instance of that class.