Sure, Quad figured it out:
He created a "custom" acknex.dll that delegates pretty much all function calls into the real acknex.dll except for a couple of functions like CreateWindowExA. He hooked those and replaced them with their Unicode alternatives. So far all lite-C functions behave the same (including the video_... commands) except that the window receives real Unicode characters.
Those are collected in the usual on_message approach.

Thanks for your help as well, Firoball!