Actually I don't think it is a good idea. You can install it and use it but there will always be those out there who don't or can't install it and thus won't see it. In the end your intent to mark a message as sarcasm won't be seen by many readers. Communication is hard enough in a medium like this without introducing something else that can result in misunderstandings. It's better to use a tag like [/sarcasm] I say.
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