> - Encoding::default_internal is set to some dummy or non-ASCII-
>  compatible encoding, which may lead to some hickups.
>  We may want to make that impossible or advise against.
>  (the main use is UTF-8 anyway)
I'd say we should make it impossible. If you are playing with dummy or
non-ASCII-compatible encodings anyway you must know what you do. So
not being able to rely on default_internal in this case would make
perfect sense to me.

> - We should think through various scenarios for output.
>  I can't think of any problems just now, I just noticed
>  the absence of considerations for output below.
I have not thought it much but if logically:
Input: default_external
вн conversion (if default_internal != default_external and encoding not
specified)
Internal: default_internal
вн conversion (if default_internal != default_external and encoding not
specified)
Output: default_external

Vincent Isambart