I've run into another problem, one that I should have anticipated.
nzbGet is running inside Docker.
Docker is designed to isolate from the outside world.
So the synoindex command, executed in a PP script, returns a "not found" message
I think there are some workarounds, but I'll have to learn more about using Docker in this manner.
For now I may just schedule synoindex to run a few times a day on the NAS.