Ah, I think I know what you're up to, Dan! But surely that works in Preview Message anyway? Just don't post it though (presumably you've already got it in a separate document anyway, which you can make amendments to, if necessary).
Or I suppose if you want to test it 'live', you could pick a really obscure topic which nobody posts in! Although you do run the risk of it being seen by some of the more eagle-eyed!