One of the problems with trying to say what would have happened in this film is that the artists were apparently just blue-skying (McQuarrie said this) a lot of the time because there wasn't a script yet, so they were providing creative fuel to the process. So even though they illustrated saucers landing, that mightn't have even been in the script, just them showing what could me done. Likewise, the asteroid starbase/dock was just McQuarrie throwing ideas out there, as the idea of an inhabited asteroid was something he'd been noodling with for some time and on other projects.
I think it's funny that people talk about building some huge model to outdo to opening of Star Wars, because that Star Destroyer model was a tiny 3' long. I've seen it in person. They built a much larger model for Empire, but it's still only about 6-7'.
You can tell at the end of the shot that there's a super wide angle lens (a tilting lens, like for the titles) used, due to some distortion, but it doesn't wreck the shot at all. The real trick is managing to light the thing in a nice-looking way while the camera is scraping right by without messing anything up; makes me think that if they had a really good rig like the one used to shoot the nacelle scrape in the drydock departure in TMP that you could have done even better, but maybe the fact it works as well as it does is serendipty, considering they spent months considering building a 40' underbelly for the shot.
What 40' underbelly? The biggest Star Destroyer model I know of is the edge of the Super Star Destroyer seen at the end of Empire
, which I've seen in person.