After installing strace from entware-ng I pinned the problem down to none of the Duplicati package's .exe files having their execute bits set. Fix, after ssh'ing in as root, is:
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find /volume1/\@appstore/Duplicati -name \*.exe -print0 | xargs -0 chmod a+x
.. after this, I could run the Duplicati service from DSM, and subsequently start the client from the main menu (the above may depend on entware-ng versions of find and xargs; not sure).