

The JSON-massage route may work. Diary items are self-contained snapshots, IDs in your file are ignored on import, and the only real catch is that re-importing a date overwrites the existing entry. Happy to drop the field-by-field shape if you want to go that route.
That said, native CSV importers for the popular apps (MFP, LoseIt, Cronometer, etc) are now on the near-term roadmap (thanks to your suggestion) as the proper path for this. If you can hold off a bit (and use one of the above), that’ll be much easier.
OIDC has been planned, but since now i have had multiple requests, i have started working on it. Will be implemented most likely in next RC release. Keep an eye out!