Leo,
Is this SharePoint in the cloud (O365)???
There is a way to do this already, but this is for the O365 version of Sharepoint (Cloud). This is where a CASB solution comes into play.
CASB solutions work with different CLOUD storage sites/applications and can be integrated with SYMC DLP (Bluecoat does this seamlessly). There are 2 diferent approaches that will be real time and close to real time. It all depends on which CASB solution you are using with SYMC DLP.
The CASB solution is either deployed as a "man in the middle" approach or an API integration with Cloud Sites/Apps. So depening on the approach gives you the real or close to real time approach. DLP would then work along with the CASB to inspect the content and either allow, block or quarantine files/content.
As far as an On Prem basis, would require using DAR scanning on a nightly basis. (incremental)
Hope this helps..
Good Luck,
Ronak
PLEASE MARKED SOLVED WHEN POSSIBLE.