I would—but not to UTF-anything. AFAIK, internally, mIRC 7 is straight UCS-2 (raw 16-bit text). UTF-16 would only be useful for handling 24 and 32 bit characters, which mIRC doesn't yet seem to support; though it's possible that i'm wrong, and it is UCS-4 internally, and just doesn't support it fully in scripts (e.g. $chr(),$asc()).
mIRC does support characters beyond the BMP through UTF-16 surrogate pairs - it is just $chr/$asc that don't. $chr/$asc equivalents can be scripted although it would be nice if this functionality were built in.