Great idea. I think it is technically possible. I imagine a 'Add Preview' button somewhere, with some parameters like which part (or complete song) you would like to have as a preview. Logic will then bounce that part to a preview audio file within the project. Or you could set a preference, so that when you close a project, Logic will:
- Never create preview
- Always create preview
- Ask to create preview
Then, the Quicklook framework could pick up this file when previewing a Logic project in the Finder.