Okay, so--it's well known that when you take the Odahviing Express to Skuldafn, your follower goes walkabout and will not accompany you the last leg of the main quest. Which...I frankly find stupidly aggravating from an RP perspective, but that's an argument for another thread.

What I'm trying to figure out now is how and why your follower is prevented from going to Skuldafn/Sovngarde with you.
I've nosed around in the Creation Kit a bit and can't seem to track down the cause. I know for other areas where your follower can't go (Nightcaller Temple, the Dark Brotherhood Sanctuary, etc.), there's a script, "defaultNoFollowDoorScript," that prevents this and spits out the "your follower cannot accompany you here" message. However, this script isn't attached to anything related to Odahviing, Skuldafn, and so on. Guess that would be too easy.
I've turned over pretty much every stone--scripts, dialogue, actors, etc. on MQ303 and still can't pinpoint whatever it is that's keeping followers away from Skuldafn. Then again, my knowledge of modding is still stuck somewhere halfway between Morrowind and Oblivion, so it's very possible I'm missing something. Anyone have any ideas?