Creating a symbolic link at the command line works fine in my experience, just so long as you don’t move the project. When creating the link, you’ll see that the project is a folder that ends with a .scriv extension. Create your symlink to that folder, and not to any of its contents.
You can create the link in a Dropbox folder, or you can move the project itself in Dropbox and create the symlink elsewhere on your hard drive, whichever makes the most sense to you.
I did this question for two reasons:
1 - considering previous projects on mac;
2- move them to other folder will break some used “document links” between mac apps - links like this > x-scrivener-item:///Users/username/filepath
My idea is create a symlink inside Dropbox to the folder on hard drive where the scrivener projects are placed (not to the .scriv folder).