Alternative solution discovered at
http://ddisoftware.com/tech/qimage-ultimate/purge-sheet-unclog-job-questions/One thing to note is that if your printer only uses matte ink when you choose matte paper, in order to exercise both photo black and matte black inks, you'd have to add two unclog jobs to the schedule: one with a glossy type paper selected and a second job where you select matte paper and only check the black ink. That way the first job will exercise all but the matte black ink and the second job will cover the matte black ink only.
With generous thanks to Mike - this has solved it!