Forums Feature Suggestions Override of a window's default font, via script

mIRC already supports this, to a degree, with its UTF-8 support - if somebody says something which contains characters that aren't in your current font, it will render them in whatever font the OS recommends.

At least that's what the 6.2 whatsnew.txt said (though I personally observed the same behavior back in 6.17, but only under Windows XP)...