I've read this entire thread and I have $.02 worth to throw in. Not to sound like an old codger, but we've been dealing with this problem for years over on CIS where the suggestions for proper behavior are a lot more difficult to access than a link at the bottom of the form for composing a message.

But we've often handled this by answering the person's question to the best of one's ability and then simply adding a comment at the end of the message like - "Hey, I realize that you're new around here so you probably don't know, but in the online world we reserve all caps to indicate shouting." Or something like that. The best results come from doing just that. Mentioning it in an offhand, casual manner.

I might suggest putting the link for proper conduct at the top of the form though. I didn't realize one was there until I was ready to send my message. A little too late if I've just flamed someone. ;}