Finally, my true question was whether Postgres would support something like worm with the performance benefits of UNLOGGED, but not the inconveniences of auto truncates.
If you can live with the limitations, one other thing you might try is storing WORM data in the filesystem and accessing it via file_fdw.
https://www.postgresql.org/docs/current/static/file-fdw.html There are a lot of downsides to this: file_fdw tables are read-only, so you have to update the external file through some other means. Also, I've never used file_fdw, so I'm not sure whether you can create indexes on the tables - and even if you can, you would need to manually recreate the indexes periodically because Postgresql won't see your updates.
George