Oops, I forgot the last two questions:

> Maybe we need better command-line control to override the defaults?

We already have -u to switch all stdio to unbuffered. This issue proposes to make stderr line-buffered/unbuffered by default, since it's less surprising than fully buffered.

> Are there precedents e.g. in Bash flags?

Well, `man bash` doesn't appear to say anything about stdio buffering.