The key is to use Aperture's export masters command, choosing the sidecar option which attaches xmp sidecar files which Lightroom can read. Test it out with a few files.
It will mean you need lots of disc space because export masters is duplicating your files - that's because Aperture doesn't have an option to save sidecars back to the original masters' folders. On the other hand, Aperture's export dialog does let you choose project or folder structure or dates.
There is another option but only if you have iView and your Aperture library is version 1-1.5, not 2. That's an Aperture plugin called Annoture which is designed to synchronize metadata between iView and Aperture. You'd run this - ie no need to export masters - and then use iView's Sync command to write the metadata into the image files. Not sure if Sync works with your exotic file formats, or if you'd want to do it anyway, but iView also has an extract metadata command which creates sidecars - sadly it's folder by folder.
I'm unsure if Aperture puts the ratings in the files, so you'd need to test with a few files - if not, copy the ratings in Aperture to the keywords and use them as the mechanism to transfer the ratings.
You also need a slightly different process if your originals are jpegs or tifs.
There are a small number of decent posts at Adobe's forum:http://www.adobeforums.com/webx?128@@.3c0599efhttp://www.adobeforums.com/webx?128@@.3c059184http://www.adobeforums.com/webx/.3bc8bc2b/1
But try with a few and then ask again if things aren't clear.