You’ve already gone beyond my experience with the API so I don’t know that I can help much there.
I thought I had seen a topic where somebody else was looking at direct API calls but I can’t seem to find it now (maybe it was on GitHub).
It sounds like this may not be an issue for you but keep in mind that by default the Duplicati server only listens on localhost so if your monitor CLI call is on another machine you’ll need to open up the server to accepting requests from there. Note that the Duplicati server is NOT hardened, so it’s strongly recommended to not expose it over the internet!
If the CLI is on the same machine as all your jobs, you should be able to set a global setting
--run-script-after call, but obviously this is less useful if you’re working across multiple machines.
And of course I forgot to include above that since Duplicati is open source you are free to pop on over to GitHub and look at directly adding Nagios/Sensu support in the source code…