I think the idea's good. But I think you should treat spoiler tag like a quick button. Just like http://, @, IMG, #, PHP, List, Quote and other stuff. And when you click on it, it asks you for an optional thing that you want to use for spoiler marking. If you put nothing there, it will just show the default style, which is: (SPOILER) but if you put, for example, Blarg, in the optional box, it shows like this: (BLARG)

After going through the optional box, you'd then get to write the actual spoiler thing that would appear invisible in the text.