Seriously, to take full advantage of SharePoints features regarding Document management, like Metadata, versioning, check-in, check-out etc. etc. you should save the document in SharePoint! This is one of the main things SharePoint is built for, and meant to be used for.
Regarding the "space" issue, saving documents i SharePoint or on a disk, you still need storage space. And if you do no enable Versioning for your documents, they will not take more space in SharePoint than on disk.
I realize that it is a matter of the pricing of SharePoint Online, but I do think that you really need to consider using SharePoint for storing the files once more. With the new features in 2013, like SkyDrive Pro where you (dropbox style) sync a SharePoint Library to a folder in Windows, the fact that the files are managed by SharePoint means that the users do not have to completely change the way they work with documents today.