Any updates on this?

I think these are the relevant messages:

IPC_ENQUEUEFILEW
IPC_GETPLAYLISTFILEW
IPC_GETPLAYLISTTITLEW
IPC_CHANGECURRENTFILEW
IPC_GET_BASIC_FILE_INFOW
IPC_PLAYING_FILEW
IPC_GET_EXTENDED_FILE_INFOW

They're all defined in wa_ipc.h

The unicode messages (IPC_*W) should be quite easy to add - just duplicate your existing code minus the string conversion (I'm assuming MM uses unicode internally).

Yes, it looks like the unicode version of the Winamp API (IPC_GETPLAYLISTFILEW, IPC_GET_EXTENDED_FILE_INFOW, IPC_ENQUEUEFILEW etc.) isn't implemented.
I reckon this will cause problems with lots of newer Winamp plugins.