I don't remember if SMF supports this, but I do know one of the many open source forum softwares I tried in the past had this. Any chance of getting rid of the edit message when you edit your post? oo We all have our "oops" moments, but I hate how it's so obvious to everyone else that you did something wrong in order to change your post. (common typos to drama) Unless you hit edit right away, you get branded with the derp of approval message that is your edit.
Most of us type "Edit:" anyway if we /really/ want it known we changed something. That is good enough.