Although the door opening itself is cut into the tube and is therefore curved, there is a beam right outside the door that will be perfect for hanging the track.
Here's a bad picture (note that the little sliver in the lower left is the top of the open door frame that this door will be covering):
And what we have in mind is basically to have the door slide right to open - the thing is that by sliding it right it will slide right off the landing and out into this atrium space over the down staircase (see how the beam cuts straight out while the curve of the tube rolls away):
So, obviously, this thing has to hold a lot of weight.
Anyway, I think these roller things and the track for them are still made, but the new ones are pretty un-decorative (Stanley makes them I think). I figured there would be a few standard models to look at on ebay. A few. So, here's the choices from ebay (including both current auctions and ended auctions):
NOTE: Wherever these things look like they are brand new or fresh painted, they were re-painted or primed by the seller. Odds are they're covering up a heap of rust.
AND THE WINNER IS...
Typical of me, I am totally wooed by the one with the most industrial looking hunk of cast iron. Also, I find the range of adjustments on these ones pretty appealing. And they come with 8 feet of track. Hopefully this plan will work - I'm pretty excited about it.