I tried doing a bit of research on this, but it's not a strong area of mine. I know there's a solution, since AutoHotkey_L appears to handle supplemental plane and surrogates just fine, and even specifically with PCRE. There's a lot of WideCharToMultiByte / MultiByteToWideChar function use.

https://github.com/Lexikos/AutoHotkey_L/search?q=WideCharToMultiByte

https://github.com/Lexikos/AutoHotkey_L/search?q=surrogate

https://github.com/Lexikos/AutoHotkey_L/search?q=0x10FFFF

...might be able to give you some ideas, anyway. AHK does a lot of what mSL attempts to do.

Example: https://github.com/Lexikos/AutoHotkey_L/..._ord2utf8.c#L41
/* This file contains a private PCRE function that converts an ordinal character value into a UTF8 string. */

Well. At least I won lunch.
Good philosophy, see good in bad, I like!