We "solved" (worked around? hacked?) this by creating a set of FunctionU macros and in some cases stubs that wrap all of the Windows entry points we use with incoming and outgoing converters. It's ugly under the hood and a bit slower than it needs to be, but the payoff of the app being able to consistently "think" in UTF-8 has been worth it.
Of course, we had to ditch resource-based string storage anyway for other cross-platform reasons, and were never particularly invested in the "Windows way" of doing things, so it wasn't a big shock to our developers when we made this change.
Of course, we had to ditch resource-based string storage anyway for other cross-platform reasons, and were never particularly invested in the "Windows way" of doing things, so it wasn't a big shock to our developers when we made this change.