I like this idea, so I tried it out. Unfortunately, I have 2 issues with it that maybe you can look into:
1) maybe you can somehow limit the height of the image shown? I have some really tall images for some posts, so showing them full height messes up the edit page.
2) I don't have anything showing up under my "pages", which also have featured images for me. I think you can fix that easily so that it works with all post types (posts, pages and custom post types), no?
thanks! again, good idea!