Append-only for mount/serve

We have --read-only but it would be really nice to have an --append-only (or something) for mount and serve. Existing files could only be opened for read. Writing to a file, once closed (and/or after the timeout?) would be locked.

Similar request: Quasi read-only mode but that allows movement of files.

Is this possible? Worth doing?

FWIW, I can accomplish something like this with a union mount with a :ro on the main data and then another place to take all new files. I can then manually (or scheduled) move those over with --ignore-existing (to protect written files). This is not ideal but is a nice workaround.