I think this would be a worthwhile feature that would be used on multiple occasions. Please do investigate.
There seems to be a spoiler plugin available here.
Since that plugin is part of the Discourse project it should (hopefully) be well maintained. I'll look into implementing it.
I do like the Stack Exchange spoiler tag though: >! It's more Markdownish.
Implemented. Write spoilers like this:
[spoiler]This is a spoiler[/spoiler]
This is a spoiler