We don't have a rule specifically forbidding fanfic. I know people often recommend and link to fanfic in the comments, and sometimes maybe posting ficlets in the comments as a result of discussion, all of which is awesome.
However, scans_daily is meant to primarily be a community for posting scans of comics and discussing them (Rule #1 of the posting guidelines is "Each post must have at least one image scanned from a comic (webcomics are fine)."). So posting fanfic and adding in some cover art for legality as an afterthought may be frowned upon. If everyone posted their fic here, the volume of posts might get unmanageable and might make things difficult/confusing for people looking for scan posts.
Just in general, I'd say a fanfiction community (or maybe a community dedicated to the characters/team/pairing that your fic is about) would be more appropriate, and you'll likely get more readers and responses. Marvel_fic on LJ is probably a good place to post.
(I'm sorry if this is rambling, but I'm aiming for helpful.)