there are multiple problems with what you suggest.
One being that irc is TEXT based, no fonts are involved, fonts maybe being used to display the text on your system and theres but thats it, its just as easy that the sender could be in a text based client that uses no fonts (assuming you consider the generic text screen text not a font in itsself).
Another problem assuming the first had been got over, is that Mirc doesnt allow multiple fonts in the text windows, so your not going to be able to use anything but the font you have currently, and thats pretty much what is done right now.
Now assuming that irc could be changed and font in use data was sent with every message, and assuming that mirc could change fonts in display and maybe in text being sent, then the last problem is with fonts themself, using fontnames is the most unreliable method of ensuring you are going to get what was sent. I personally have seen 4 different versions of the same font, i well admit the differences were mild, and unrelated to what would be the feeling meaning to be passed by a message using this font.
There is some font matching system involded in windows, im not really sure how it works, but from the results i have seen from it its pretty crappy (imo).
PS : sorry for the missdirected info before on $window() but i thought you were having trouble detecting your own windows fonts etc etc.