Oh, for what it's worth -- here are my hacky Amazon plugins (they work for me, mostly, and if anything they're a starting point well outside of Calibre's internals...). Haven't had time to polish them since my initial posts here, but feel free to take a look.
Amazon Book Covers.zip
Amazon Book Details.zip
You should probably disable the original amazon plugin if you intend to test them. They won't collide, but there's probably no sense in querying the same source twice.
When you customize the plugins you'll need to enter an Amazon API ID/key and secret (
sign up here), and then select the language domains that you want to use from the list (and arrange them by dragging -- they should then be queried in order, from top to bottom). Haven't really tested that part though
Those plugins work slightly different from the original Amazon one, in that they use author/title data for queries if ISBN is blank, so you can actually get multiple results from Amazon.
A shame Calibre's cover download feature doesn't present you with a list of results so that you can choose the cover you like (like it does for other metadata), because at the moment it's a bit of a lottery -- I believe you get the first match, which might not be what you like... I wish I had more time to see if it's something simple enough for me to add.