Sounds like a fun plugin to code, @jackie_w, enjoy
Regarding the actual use case -- sharing books for debugging purposes, I think that simply putting the ebook file into a password protected zip file and sharing the password only via PM would do the trick. Ideally, some UI could be added to the MR upload attachment window to upload a "password protected attachement", the user then specifies the file to upload as normal, and also a password, the MR server then zips up the file in a password protected zip file which becomes the actual attachment.