In this document, the use of the attribute rel="NSFW" will be called Rel NSFW
in short hand.
Obviously, a link with Rel NSFW is valid HTML. However, not all elements can have a rel attribute.
For example, this paragraph is Rel NSFW and fails validation. I can also add an image that is Rel NSFW and it will fail validation.
A better method would be to use the title="NSFW" attribute, or follow the microformats crowd and use class="NSFW". This solves all the problems you need solved and is equally or more styleable than using rel.