How to create a lightweight backup for an iMovie project (i.e. without all the videos)?
This may not be the best workflow, but here is one that works.

To create a lightweight backup of a project:


First, create a backup of your Library in case things break.
Ensure you're only using a single library for each of your projects. Otherwise it'll be harder to backup only the project you care about.
In Finder, show package contents of your iMovie library.
Delete all the "Original Media" videos. You can also delete other large files such as "Render Files".
You should now have a lightweight backup of your iMovie project without all the videos.
Ensure your backup can be restored by duplicating it and following the steps below.


To restore a lightweight backup of a project:


Open the iMovie Library for your lightweight backup.
You'll see that a bunch of videos are marked as missing.
Reimport your original video files, and everything will just work!


Another thing that will make this entire process easier/possible is making sure not to rename/edit/delete your original video files.