mIRC use utf16 internally, not UCS-2.
Having a workaround for scripting is a good idea until it's built-in, but keep in mind, as Khaled mentioned, that this is touching mIRC as a whole, the highlight features has to support that as well.
Perhaps my understanding of UCS-2 was incorrect. But if mIRC used utf-16 then $len of any unicode character encoded as a pair of 16-bit values would return 1 rather than 2. The fact that you have to manually encode the surrogate pairs indicates that this is not genuinely the case.
However, my offer to share my starting point for a Unicode script still stands.
