It's not much, but maybe another 'notice' type text (as when you select to send to all in realm) could be put in place so that, when sending OOC, a question is raised as in "are you sure this should be posted in game, or should it be in the forums?" and give a link to the forum.
I think this solution, or a variation thereof, is the right course. We'll never stop occasional meta-discussion or meta-whining in game, but we can make it so they have to think about it before hitting that send button.
