Is it really possible to update the fid-files without destroying the database that the player is using?

Yes and no. You can update the fid files themselves, but during play the player uses a separate database which is only reconstructed from the fid files during a synchronise operation. There's no way on the player itself to cause a database rebuild.

Read-only LDAP plus streaming sounds like a neat hack, but LDAP is not the way to go for read-write access to the song tags.

        Peter