That's scary re-reading my old posts. I've just had a quick look and
https://aifftagging.blogspot.co.uk/ seems to give a decent state of how the tagging is now don. From that it seems to follow using ID3v2 tags rather than the native tag fields (which sort of makes sense.
I found
http://samplephotovideo.com/2015/12/download-aiff-audio-interchage-file/ which seems to have both types of tags and having a quick play around, it seems like winamp / wacup as-is is only reading the native fields rather than the ID3v2 tag as other players / editors are doing.
The handling of 'wav' file types sadly hasn't improved since that other forum post but it's more down to time to get something done that prevents even just having it follow the more popular formats (e.g. mp3, flac) and being handled via the unified alt+3 dialog. So this s something I'll definitely keep in mind as down the road I want to do some wider changes when it comes to tag handling to deal with some long standing annoyances I've got with Winamp + tagging in general.
-dro