Right, the unicode article by jaytea isn't so clear about it (and it's pinned so that's kind of mIRC claims), there's nothing about this, only /var and /echo are used to produce such characters in the article and no mention that you can't use this anywhere.
That being said, if it's only the rendering function which happen to show these chars, (compared to $replace's handling being different, here), then it would also render with $regsubex, given it were correctly replacing just like $replace is.
Which makes it a bug imo, $regsubex shouldn't be fixed because it makes mIRC's support for unicode plane 1+ but because it's at least inconsistent, if not just wrong, with the way it handles surrogates.
