Okay, so I posted http://www.gamesas.com/topic/1419864-do-we-have-any-specific-credits/ on the F3 General Discussion forum, now I've decided to post it here, but extending it to NV too.
Finding an actual list is pretty much impossible, but I've scavenged some information from interviews, you can see it if you click the link, but I'll sum it up here too.
In http://planetfallout.gamespy.com/articles/interviews/631/Interview-With-Jeff-Gardiner we can obviously learn that Jeff Gardiner is the lead producer for all the DLCs and that:
And here's a http://www.nma-fallout.com/forum/viewtopic.php?t=51317 to a thread from NMA where they say that Brian Chapin wrote Mothership Zeta according to an interview. It seems most of the links in that topic lead to nowhere. I don't quite understand this bit however:
Okay... but wasn't Emil Pagliarulo the lead writer? Or did he just write out a rough sketch for the main storyline and only did a few actual quests for it and the majority of his writing is something different? I'm confused.
I also could not find any real information as to who wrote Point Lookout. I remember reading that PL was basically the designers being given free reign.
And do we know anything about who did downtown DC or who designed which dungeons (I'm especially curious about the Dunwich Building and the inside of the mothership).

Also, about NV... I know most of the stuff was written by Chris Avellone. But Nukapedia (that's the wikia site, right?) indicates that he only worked on two of the DLCs: Dead Money and OWB. At least that's what the "Involvement in Fallout series" says. But then the article says that he was lead designer on all DLCs. What, that doesn't count as involvement? I especially miss Lonesome Road since Ulysses was written by Avellone, wasn't he?

Well one way to get the answers you want might be to check out the Game Guides. I have one for Fallout 3, I'll check it out but it was before the DLCs.

They should probably lower Brian Chapin's position of writer. Make him in charge of some minor quests or something, and then they should put those two Pittsburghers as lead writers.

Well my game guide was a bust. But that doesn't rule out the other three. Maybe one of them will tell you.

I don't have the game guides. How many are there?

To be honest there wasn't really any writing in MZ, it was only the logs that bothered people and it is entirely possible that Brian had nothing to do with those. I still don't get it though. What does it mean that he gave us the main questline? Does that mean that Emil only wrote it but didn't much participate in the MQ's execution? That would explain the massive amounts of plotholes. It is very much possible that Emil wouldn't have given the Enclave as much firepower as the people who actually did the quests.
And I've read somewhere that those two fellows left. But they can still be found in the Skyrim credits though. And we don't know what they mean by the "bulk of the writing". They might have meant that they were responsible for dialogues only and the actual storyline was written by someone else. This is the main reason I would prefer a list. I'm thinking about asking NMA, they seem to know the most about Fallout 3, which is only natural since they hate it the most.

If I had to guess I would say Emil's lead writing role meant that he was connected to everything involving writing and oversaw it. So this Brian guy writes up the main quest and he has to give it to Emil. Emil then reads it over, puts some of those signature "cool" things in, then proceeds to check the writing of another quest.

This Brian guy probably had nothing to do with Fallout 3's "cool" things and plotholes. He is probably a nice guy.

I don't know I think the lead writer is involved with writing out the MQ, but Emil is only credited with the designing of one quest, which is the Tranquility Lane one. I would guess though that given his past as a journalis that Emil would be responsible for writing some of the terminal logs. He probably didn't write the MZ logs though.
Also, fun fact: Brian Chapin is the dev responsible for the cliff racers and their numbers in Morrowind.

I also forgot to mention in the first post that I'm talking of Chris Avellone's page on the wiki. Just saying in case it wasn't clear.

I believe there are four game guides for Fallout 3. Bethesda really knows how to milk money out of fans. I got one but I am not going to buy the other three, unless their price drops, or I find them at a yard sale or something.

Re-posting bits here at Nas92's request:

It's not specific on their involvement but this http://www.xbox360achievements.org/forum/showthread.php?t=147931 tells us that Point Lookout was the "brain-child" of Joel Burgess and Nate Purkeypile. They might have done the writing and/or quest design.

This http://www.gamebanshee.com/forums/threads/fallout-3-point-lookout-mothership-zeta-dlc-interview.109846/ gives us someone to blame for pitching the idea of Mothership Zeta in the first place, Lead Artist Istvan Pely.