Well, I'm pretty sure the bg color for the Quote box is always the same, so you could write spoilers in quote tags and change the text color like this:
[quote][color=#717171]spoiler text[/color][/quote]
Blah blah blah Ban's a martian from hell!!!!
That looks like it will work fairly well. I guess I could add that to a section to show how the spoiler system works.
The only problem is making sure everyone knows to do that. Is there a way to make a new tag that does it automatically? I dunno. I'm not an admin.
If I could do that then I would have done it a very long time ago. Unfortunately, SMF forums are fairly limited in what they can do.