Upon further investigation, it appears ProConnect currently has the ability to mask SSNs on 8879s under File Return / Print / View/Print eSig Docs.

Under Print Options in the center of the screen, there is a slider button to "Mask SSN and add watermark". There is currently not an option to mask EFINs though. 

Unfortunately, Masking SSNs/EFINs is not currently available for the DocuSign documents in this same area (File Return / eSignature / Client Authorization eSignature). 

So, the ability to mask confidential data (SSNs, EFINs) is available in ProConnect. It would be great time saver if it could be implemented for the DocuSign eSignature documents (8879s and others) as well. 

Until anything gets updated in this area (which doesn't look likely at this point), I'll have to keep using Adobe to manually redact EFINs/SSNs from the DocuSign eSignature 8879 forms and other documents (like others have been doing) before sending them out to clients.