Thanks for the bug report, unfortunately I have not been able to reproduce this issue here. When I /echo that character, it displays correctly in mIRC. This is probably related to the
recent discussion on Unicode fonts.
As for using Unicode in nicknames, I am also unable to use that character in a nickname. When I try to do so, mIRC sends the correct UTF-8 representation of that character:
-> efnet.xs4all.nl NICK :hεllo
<- :efnet.xs4all.nl 432 arnie hεllo :Erroneous Nickname
The server replies with an error message indicating that those characters are not allowed in a nickname. Is that the message you are seeing?
Most servers allow different sets of characters for nicknames and channel names. If I try to use the same character in a channel name and join channel #hεllo it works fine.