My solution to this is to just be a little patient.
I do the same thing, but I leave my unison downloads going overnight, so I use Hazel like this.
Set downloads folder to check for .nbz files and if the creation date is the same as todays date do nothing, if it is yesterdays date or older, open with Unision then delete the nbz.
Check the creation date of the .par2 file without + in the filename in the Unision dlownload folder. If the file is older than, say 2/3 hours, the open with MacParDeluxe, run rules of folder contents to deal with the extracted file.
You can also set split&Concat / MacPar to trash parts when they have finished with then, and Hazel will clean out the trash periodically.
Result: an overnight downloading session (starting just after midnight), nicely sorted and cleaned up in the morning
I hope it helps others out.