How do I open older .imovieproject files (they're actually folders folders) in iMovie '09? I don't want to import the assets of the old project, I want the actual project to open on iMovie '09.
You can't, actually. iMovie '08 and subsequent versions are a completely different program from iMovie HD 6 and previous versions.
However, the last released version of iMovie HD does still run on modern Macs. It used to be a free download for users of iLife '08, but Apple has since removed it (IMHO, to "encourage" power users to upgrade to Final Cut Express, since iMovie '08 was utterly useless if you happened to have been one of the few who knew what they were doing).
It is still possible to download and install iMovie HD through some avenues of questionable legality.
Had this question myself since I couldn't get the folder to be registered right due to the incompatibility. Then I realized the "Media" folder in my .IMovieProject folder contained an MOV file I could just re-use in a new project. Shame to loose my minimal editing, but at least I have something.