"Proprietary MP3 tags" is nonsense, the Amazon support person who told you that didn't have a clue about what the company is doing.
Just think: you say yourself that the metadata show in some Open Source players, so it's obvious that there's nothing "proprietary" going on.
My bet: some unusual fields used or maybe they use ID3v2 tags and mt-daapd only understands v1. Indeed, the first two google hits for "mt-daapd id3v2" query indicate precisely that: http://www.google.com/search?q=mt-daapd+id3v2
"Proprietary MP3 tags" is nonsense, the Amazon support person who told you that didn't have a clue about what the company is doing.
Just think: you say yourself that the metadata show in some Open Source players, so it's obvious that there's nothing "proprietary" going on.
My bet: some unusual fields used or maybe they use ID3v2 tags and mt-daapd only understands v1. Indeed, the first two google hits for "mt-daapd id3v2" query indicate precisely that:
http://www.google.com/search?q=mt-daapd+id3v2