LISTSERV will decode quoted-printable or base64 requests sent to the
LISTSERV address (old versions may not do this). I don't see any reason
why accented characters would be a problem. DBCS characters are another
story and I would not recommend their use in passwords, because LISTSERV
has no knowledge of DBCS outside of the web archive interface and a few
other places, and will process them as a string of regular single-byte
characters. Some of these single-byte characters may have special meaning
(I don't know for sure but I wouldn't be surprised if a SPACE were
possible) and you may not get the desired results.
Eric