If a link is missing from my file, rather than hunt down the link, I create a new file, and embed the file with the missing link in it. Once it's been embedded I have the high res raster image, just not necessarily with the same name. Much like photoshop tells you raster info will be used for fonts, or missing links use raster info it would be great if Illustrator gave you the option to embed. It may only be possible for images with full PDF compatibility, but it'd be a great improvement!