mIRC actually already does this :-) It preloads a number of known unicode fonts and searches them in the order: Microsoft Sans Serif, Arial Unicode MS, Tahoma, Arial, Lucida Sans Unicode, Code2000, Fixedsys Excelsior 3.01. However in mIRC 6.35 it was still possible for a bad font to sneak in at one point. I have optimized the font-linking routine to try to prevent that. Once I release the Unicode beta for public beta-testing (in the next few weeks hopefully) we can see if the change resolves the issue for you.
