FWIW it also works fine here under Windows 7.
Your workaround seems like a pretty complex implementation for what is essentially a temporary bandaid to deal with your lack of proper unicode font (and unicode *fonts* on your system).
Also, your scripted suggestion couldn't work. The data is the same data in any font, so using $fontlink wouldn't do anything to the %input_text. The only way to give advice to mIRC about font linking would be to provide some complex switch to mIRC about which glyph should be linked as which font, which, even if implemented fully, would still require you to (a) figure out which characters (and their respective positions) need to be linked as which fonts and (b) override any event where mIRC echo's text and substitute your own /echo. If that sounds extremely complex, it is.
My suggestion would be that if you're using Arial ..don't. Just use Arial Unicode MS. As you said, there are proper unicode versions of most standard fontsets out there. If you want proper unicode support, simply use those- not the non-unicode versions. There's really no reason to choose Arial over its Unicode counterpart if you have it on your system. It will also save mIRC the trouble and CPU cycles of having to link glyphs from the font you should have chosen.
