After 1 hr of puzzling, fiddling and searching I found the solution to this problem in one of my own doc files from 2008:
"Für wv keinen Eintrag bei Waveform Decoder machen, um den Dateityp in die Liste zu bekommen, sonst spielt Winamp keine Wavpack Dateien mehr ab!"
In Winamp under Options->General Preferences->File Types there's a list of associated file types, where you check/uncheck for which file types Winamp should be used as the default player.
The suggested way to have a new file type included here is to goto Preferences->Plug-ins->Input->Nullsoft Waveform Decoder and add the new entry there, like aif;ac3 etc.
Just don't do that with wv : Winamp will no longer play Wavpack files!
Just wanted to share this knowledge, and perhaps next time I'll find the information quicker here than this time. I don't think this is really a major problem anymore, because (I believe) that Winamp has by default the wv entry in the file type list now. It just used to be a standard routine for me to customize the list directly after installation and I forgot about this updated state of things.
Regards and thank you for Wavpack!
Associating the WavPack extension to in_wave.dll doesn't make any sense since the correct input plug-in for it is in_wv.dll, and the file type becomes available in the Winamp General Preferences automatically once you install the relative plug-in.