Since the subject of the question is in comparison to Emoji, I presume you do actually mean in the sentence or in a way that provides additional expression related to the writing itself and not just stuff alongside the writing.

However, you might be interested in marginalia, which is the content that they wrote and drew in the margins. If that is the case, a good start is a response /u/searocksandtrees about marginalia, specific themes, and tangential questions [here] (https://www.reddit.com/r/AskHistorians/comments/4beef7/comment/d18f6pj/)

You may be interested in my recent answer concerning ocular Os in Old Church Slavonic.