What I envision would be pens or other writing implements being added to the game, and they would have the function of adding a 1-2 line message to existing documents - maybe it's signing on a dotted line, maybe writing in the margins, etc. (possibly with the option of adding more lines on a blank back).
Additional functionality could be to create a "signature" command, that would automatically add a character's name or other custom signature/autograph to a document. This would be something that would be authentic and unique to the character, so for example it could be identified as actually being signed by a particular celebrity. At the same time, means could be added to forge such signatures, and forensics skills could be used to identify a forgery from a real signature.
If this additional functionality is possible, i.e., verifiable signatures, I think it opens up some interesting possibilities. Signed documents can be created and/or forged, they can be used to enforce contracts, autographed memorabilia can become high-priced collectors items (and likewise spawn a black market of forgeries), coded messages can be scrawled on flyers for those who know what they mean, etc.
Again, I have no idea if this is even possible, but it's just something I thought of that I feel would be a useful RP piece.