mIRC is not a unicode application, so you would need to make sure that the text is in UTF-8 format.
I followed Horstl's directions and they worked fine for me. Are you sure that the text in the quitmsg.txt file was in UTF-8 format? I created quitmsg.txt using Notepad under Windows XP and then saved using the UTF-8 encoding option.
Another method: you can paste the UTF-8 text itself into the Quit message box. This worked for me as well.
If you open a debug window by using the "/debug @debug" command, you can check whether mIRC is sending the Quit message in the correct UTF-8 format.