You can use a postprocessing script to do that. Note that scripts receive a bunch of useful info in the form of environment variables in addition to the 8 command-line params listed, including the full path to the nzb.
If $7=0 and SAB_CAT <> "" mv SAB_ORIG_NZB_GZ /mnt/tdrive/NZB_backup/SAB_FILENAME else mv SAB_ORIG_NZB_GZ /mnt/tdrive/NZB_backup/SAB_CAT/SAB_FILENAME
(first test the thing worked, then if category empty, just chuck in backup dir and if category NOT empty then move to a sub-dir with category name in backup dir)
But presumably that means I have to switch off NZB backups in the SAB config... plus this would be a bash script rather than python