Automatically converting known abbreviations like "plz" is reasonable. Proper format and proper spelled language helps to get the information.
But how to make sure you replace the proper usage and it's not e.g. a variable. Pure automatic replacement might be difficult.
At least there could be automatisms to help people cleaning up, aka Wikipedia bots or search and replace across multiple posts for users with > 10k rep.
And there would have to be a list with abbrevs and their "support" what it really is. And typical known mistakes, e.g. http://www.yourdictionary.com/library/misspelled.html would have to be added, too.