Thought of some more. There's Pete Townsend's failed Lifehouse project, his overambitious follow-up to Tommy. Of course, it wasn't a total bust: Who's Next came out of it. Decades of AOR oversaturation wasn't enough to sour me on this album. Ahh, I remember the TT days when I'd add, "And this is even on-topic 'cause Giles sang 'Behind Blue Eyes.'"
Randy Newman did a musical version of Faust. I've never heard it, but I like Randy Newman a lot and Christgau loved it. One could probably make an argument for Good Old Boys, too.
I'm currently listening to Phil Kline's Zippo Songs: Airs of War and Lunacy. I don't know if it's rock, but it's certainly a song cycle ("Donald Rumsfeld is the new Dylan!" - well, maybe not; click the link to see what I'm talking about) and Kline plays a lot o' guitar. It's closer to the art song tradition (in concept, not sound) than to opera. The singer, Theo Bleckmann, is great! Gorgeous voice. The reviews aren't kidding about his range either, but the great thing is that he shows off that range without showing off at all. I don't know how much of that is him and how much is Kline's writing, but whoever deserves credit deserves a lot of it. No Whitney/Celine/Mariah dog-whistle showing off. Every high, low and mid-range note is organic to what he's doing, not jammed in there for cheap effect. I heard about it when Phil Kline was on Soundcheck.
And the more I thought about it the more I became convinced that not only is Funkentelechy versus the Placebo Syndrome a rock opera, Parliament's whole output from Mothership Connection through Aquaboogie is one big concept album/rock opera: the tale of Starchild, joined later by Mr. Wiggles the Worm (and his ladies Giggle and Squirm), fighting Sir Nose d'Voidoffunk in the Zone of Zero Funkativity. "You will dance - you must dance!" "I will never dance! Oooh, I'll get you for this, Starchild!"
ETA: My sister had the limited-edition, yellow vinyl copy of Grand Illusion. She had the regular black vinyl Pieces of Eight... "The jig is up, the news is out, they finally found me: the renegade who had it made, retrieved for a bounty." (sorry for the earworm... heh heh!)