It's weird that you say that after claiming/knowing mirc doesn't support it.
Autohotkey is not relevant, there is no denying that an application can correctly provide a way to user to use/generate these chars and provide, let's say, a string library, like mirc is doing for the BMP. The issue is that, as mentioned, all functions in mIRC deal with utf16. Suppose $chr() is extended and nothing else is done, given that the character in this thread is code point 128287, $len($chr(128287)) would, just like it does right now with combining two surrogates, return the number of element in the array of 16bits, so the two surrogates, which is problematic.
The solution would be to use 32 bits, but there, Khaled already decided against it when converting mIRC to be unicode compatible by choosing the 16 bits design.
I'm also sad that we can't deal with characters others planes in scripts
