A few things:
The extra effort it takes to decode rot13 is an incentive to try and figure things out on your own. I really like this feature, as anything that's only one click away is a huge temptation to go and click it, and ruin what I might have really enjoyed finding out on my own. Rot13 is one of the few instances that allows my akrasia to benefit me.
If you do end up using another web service, please operate on only the alpha characters, not punctuation. Basically, still rot13 but with the links. ᆲマミヨモレヘフヒミレムワヘニマヒリミラレヘレ is much more jarring on the eyes and attention-grabbing than guvf fgevat bs grkg. But more importantly, it lets you keep markdown when you rot13 something. If you want vgnyvpf, you can write *italics* directly in. If you obfuscate punctuation (especially spacing) than it is absolutely impossible to figure out where to put the markdown after you encode it.
Honestly I'd skip the middleman and go straight to spoiler tags. Here's the CSS code for spoilers tags on reddit. Lesswrong uses the reddit engine so this would be incredibly simple to implement. Just a matter of getting site consensus and showing it to whoever runs the site. (Lesswrong admin, can you implement this please?) Alternatively, you can hijack the link markdown: Hover for a major Spoiler. This looks like [Spoiler](http:// "Spoiler text here.").
Those polls are awesome.
rot13 is... (Results)



A quaint internet tradition that I quite enjoy
So annoying
Easy to use because I have Leet Key
Vapbzcerurafvoyr
In November, DanielVarga complained about rot13.com as a way of hiding spoilers, and suggested some specifications for an alternative web service, with an algorithm that can operate on any character. I whipped up two, one using a bitwise not and the other using a password. These have a usability advantage over rot13.com: they automatically generate a decode link, so that people who don't have an add-on such as Leet Key (get Leet Key! or d3coder if you're on Chrome!) on whatever device they're on can still use them. The advantage over using a site like pastebin is that all of the text is encoded in the url, so if makefoil.com is down or you're offline you can still extract it with a bit of work. Both algorithms are in unobfuscated javascript; feel free to host your own version.
Recently, I had the thought of doing a similar thing with polls. This site doesn't have built-in polling functionality, and the project to create it seems to have stalled. People generally solve this by creating a comment for each option, asking people to upvote the comment of their choice, and asking people to downvote another comment to balance the karma. It's elegant as these kludges go, but it's still visually confusing in a comment thread, clogs the "Top Comments" lists, and somehow rarely ends up balancing correctly.
Here's the site I used to create the above poll. Those vote buttons will turn into vote counts once you vote (you may need to refresh). The site outputs poll syntax in two formats: html, which you can use by creating an article and clicking the html button on the top tool bar, or markdown, which is for comments. The markdown display is a little different because the Less Wrong functionality for having an image be a link seems to be broken; the image gets double-encoded or something.
I hope you'll consider using markdownSpoiler or passwordXor when sharing secret info on the next Harry Potter and the Methods of Rationality chapter, and getting input with a makefoil poll.
Notes: