Yes, it’s working, more-or-less; I’m not clear on whether the above is a problem in packaging of 184.108.40.206 (exe files should have execute permission) or in DSM’s handling of the package when installing it, so can’t confidently report it as a bug. I was mainly reporting my experience in case anyone else ran into the issue.
As far as “working, more-or-less” is concerned, it starts under DSM, but I can’t use the GUI from within DSM at the moment (the Duplicati GUI comes up, then says “you are not logged in”). However, after starting it under DSM, Duplicati will work when I go to http://my-nas-ip:8200/ from a separate machine - I have half a suspicion that this is because I said “yes, this is a multi-user machine” and entered a password, but haven’t tested that aspect thoroughly.
I’m also working through some file permission issues (successfully made a local test backup and restored a file, but having restarted, can no longer access the backup); that’s probably down to my initially starting it by hand over ssh while testing though.