If the arguments to system calls are going to be UTF-16 encoded strings, then the call should explicitly use the Wide character version of the function (produced by appending a W to the function name). This way, Juce will still compile and function correctly even if the project settings are not set to Use Unicode:
e.g.
HMODULE h = LoadLibraryW (optionalDllForDirectLoading.toUTF16());