My trusty MacOS advisor has pointed out that since QTR installs as a printer driver in MacOS, defining custom paper sizes is simple, much like in Windows.
So let me take this a step further. What I'd also like to do is change the order of the inks in the PPD, because the inks are in non-standard slots in the printer, and obviously I'd also need to change the top line of the quad files to match. So my question about whether the PPD file can be edited on MacOS still holds.