You'd probably have to add in your own markup so that something like ~~SpoilerTextHere~~ would get turned into something like <span class="spoiler">SpoilerTextHere</span>, then add .spoiler class to your stylesheet (or probably better in futaba_style.pl since you probably want it to apply to multiple stylesheets), in which the text is black against a black background and the text is white on mouseover.
My guess is that you won't have an easy time doing it. I wouldn't.