the main problem is that the conversion is applied to
every channel
disregarding specified encodings.
For UTF-8, mIRC extracts the target channel/query and the :message portion from PRIVMSG, NOTICE, PART, TOPIC message that is about to be sent, and encodes the :message based on the UTF-8 setting for the target channel/query.
mIRC could do the same for Japanese but, if it's more convenient, encode not the message, but the whole string. But
channel settings should be taken into account! that's the main wish

another wish is to take
network names into account, too. i've got two channels named '#Japanese', one of them is ISO-2022-JP and another is UTF-8, making one of them read-only for me.
(as for me, i can have a non-english nickname and type Japanese in a utf-8 enabled channels on some servers that do not specify 'CHARSET' at all in the numeric 005. but both my nickname and message are in Unicode)
(strings in ASCII and a string in ISO-2022-jp (JIS) do not differ if they contain latters, punctuation, etc ASCII stuff, much, if not all of the letters used in the IRC commands (see
byte map - these symbols are the same in Shift-JIS as well). Hence, as i assume, the 'whole string conversion' will result in the same as would the 'partial' conversion of messages, and nicknames and channel names.)
(regarding the channel names and nicknames in ISO-2022-JP. well, haven't seen JIS nicknames ever, but regarding the channels, such do exist.)