Perhaps I should just post what I know. So Miiverse drawings and posts that are saved in offline mode are stored in a file called largeStorage.bin, you can get this file by dumping the SysSave data for Miiverse (I used JK Save Manager to get it). Anyway, in hex editor, the offset where the first saved post is contained is at 000AB000 if anyone wants to check it out. Also, I should probably mention that Miiverse drawings only accept the colors white and black from what it looks like, So no colored images. If you're mainly interested in editing this file for image uploading, I would imagine that the process for that would be similar to this.
Now I would continue, but I'm really tired right now. The post data is not hard to understand, it's extremely easy to read. If anyone has any questions, ask away.