I have no idea how doable this is, but here goes... The basic idea is to have a "save draft" option, which would not visibly submit the message but a user could navigate away from the page and return at a later date to modify as desired before posting or sending the message, or scrapping it. This feature ought to be standard for PMs at the very least, because they're often used similar to email, and to my knowledge all email systems permit the saving of drafts. Setting up a "save draft" system for forum posting might be a bit trickier/impossible. I don't really know because tech is not my specialty, but figured it couldn't hurt (much) to ask.