Yeah, this is quite late. Here's how I think you can do it. Run Show Package Contents on the .rcproject file. Find the file simply called "Project". Make a backup copy, and then open it with TextWrangler or similar program*. Search for the path in the file, fix it, and save. Try it in iMovie again. (The version I have is 9.0.4).
* TextWrangler allows you to edit binary XML files, which is what Project is. I'm sure there are other programs that can do the same. This one is free.