Via the good folks at Podcast the Ride, I was pointed to this video from The Donny and Marie Show: a musical spoof of Star Wars. I haven't listened to their podcast about the video yet, but I'm looking forward to it (it's behind a patreon paywall – yes, I'm a paid subscriber).
The episode aired on September 23, 1977. The movie was released in May of that year, so this segment was broadcast after a summer of record-breaking box office for the film.
The jokes are so bad you wonder if they're actually jokes, until the laugh track lets you know that they are. The songs are eye-wateringly corny. This makes The Star Wars Holiday Special look like Andor.
The show was apparently made with the permission of George Lucas, so the producers got access to some actual Star Wars establishing shots of space ships (but also used their own cheesy and puzzling establishing science fiction shots, and even a shot of an Apollo-era rocket). And they got access to the R2-D2, C-3PO, Darth Vader, Chewbacca, and Stormtrooper costumes, which must have made this of at least some interest to frenzied, content-starved Star Wars fans at the time.
It's got very, very 70s guest stars: Kris Kirstofferson (as Han Solo), Redd Foxx (as Obi-Wan Kenobi), and of course Paul Lynde as an Empire villain.
Maybe watching Donny and Marie as Luke and Leia is where George Lucas got the idea that the characters should be siblings.