...I offer the following fics. It's been a while since I've watched any Supernatural, so I can't necessarily judge the level of OOCness in them, but hopefully at least one will meet your selection criteria.

As a warning, all of these are rated M.

1) http://www.fanfiction.net/s/8895171/1/I-hadn-t-always-been-like-this
Author: HardcoreSparkleParty PM
It's Highschool, and Sam Winchester hasn't always been like this. He didn't even know about these types of things until it hit him. AU - LUCIFER/SAM - RATED M
Rated: Fiction M - English - Drama/Romance - Sam W. & Lucifer - Words: 738 - Reviews: 4 - Favs: 2 - Follows: 1 - Updated: 01-11-13 - Published: 01-10-13 - Status: Complete - id: 8895171

This one has possibly the largest character ruptures I've ever seen, although it is a highschool AU (apparently). At least it's blessedly short.

2) http://www.fanfiction.net/s/8802616/1/Playing-with-the-Devil
Author: drade666 PM
Drade (my OC) walks in to find Sam being molested by hallucifer while he sleeps only to join in but how will his girlfriend Teresa (my stepsisters OC) react to her boyfriend being taken advantage of?
Rated: Fiction M - English - Supernatural/Humor - Sam W. & Lucifer - Words: 1,730 - Favs: 1 - Published: 12-16-12 - Status: Complete - id: 8802616

Has a couple of OCs in it as well, in case you wanted to include some Assassinations.

3) http://www.fanfiction.net/s/8620735/1/Wing-Ecstasy
Author: EliwaFlynn PM
For Samifer Week 2012. Lucifer has always told Sam that he is still an angel. Curious, one day, Sam wants to see Lucifer's wings, which arouses a lot more than Sam expected. (Smut, okay?)
Rated: Fiction M - English - Romance - Sam W. & Lucifer - Words: 2,878 - Reviews: 5 - Favs: 26 - Follows: 2 - Published: 10-18-12 - Status: Complete - id: 8620735

4) http://www.fanfiction.net/s/8145461/1/Monogamy
Author: mandrake-o PM
AU where Sam is a senior at Stanford and Lucifer is the human history professor he's been having an affair with who is not happy when Sam breaks things off to be with Jess. It's a mildly twisted love story. Sam/Lucifer slash.
Rated: Fiction M - English - Romance/Suspense - Sam W. & Lucifer - Words: 11,813 - Reviews: 10 - Favs: 43 - Follows: 3 - Published: 05-23-12 - Status: Complete - id: 8145461

...good luck. I wish I could help (for when you do get permission), but I don't happen to read that particular pairing and don't really want to start.

I am glad to see you want to write SPN missions, though. We could use some more of those.

~DF

In addition to permission, it's sort of... really HARD to find a fic that fits criteria that you set up. Badfic first, THEN start planning out a mission. Believe me, I know how difficult it is. But do you know how hard it is to find a fic with an OOC Arcade Gannon? Pretty dang hard!

So, just look for bad fic first, and then plan your mission around it. If you can find a bad fic that fits your criteria, great! Go for it! But believe me, it is really hard.

Don't you need permission before you start writing missions?