Hi dro,
Oops, I read that post without looking at who it was from. I assumed it was from you since you're the only one who could answer my question.
I've been using Winamp with the plug-in thinktink wrote awhile back to embed albumart. As you say, mp3tag (among other apps) can also be used to do this.
Anyway, take your time with providing a native WACUP solution. I understand this will not be a quick and easy thing to do since you want to consolidate all of the metadata writing for all the native plug-ins (which is the logical thing to do).