You can move the documents once added to MDG's attachment list into the optical archive by creating an code enhancement and add the link to the optical archive document as a link in the same Attachments tab of the Change Request.
But I would assume that you would want to store these links within the Master Data section instead of at CR level if these are to be seen even after CR is activated. As after activation, a user displaying the object will not see the CR section. If this is the case, you would have to add a custom table entity which links the object ID (material/supplier/customer) with the optical archive link to the data model and show this on the MDG UI so that the user can navigate to the document later.
Regards,
Kiran P