Getting LAME to handle Unicode filenames and tags was harder than I though
Apparently the current LAME v3.98 release version doesn't support Unicode at all (at least on Windows), so I had to compile a LAME v3.99 Alpha build from the sources.
As a result MP3 encoding should work now. But currently only WAV and MP3 files will work as source, as the decoders aren't "activated" yet...