I have a very similar situation where I actually have an entry way that is framed for a 28" door that goes into my basement. I have removed the door and jamb. I recently finished my basement and was having trouble getting furniture etc. into the narrow door width. Removing the jamb gave me a few more inches making it just wide enough. Now that the basement is finished, I'd like to cover the entry way without narrowing the interior width of the door, i.e. no door jamb. I've considered something like a storm door that mounts on the outside of the door, but that would be tacky for an interior door.... Any ideas if you didn't want to get a pre-hung door AND it was sturdy enough because it was framed for a door?
Just don't get the door that swings both ways (mrc59's post) - don't want to knock someone down the stairs by accident!