Has anyone ever heard of successfully keeping a moorish idol?

Yes. It is difficult but can be done. The fish needs a large tank with high water quality and a LOT of live rock. They also need specialty angelfish food that has bits of sponge in it to subsidize their diet. They key is the live rock though, which provides natural forage ground. Good rock will have lots of sponge and inverts that the idol will like to munch on.

They, like many "expert level" fish usually need larger tanks, high water quality and a special diet. The diet is usually the hardest part, but as long as you know what you are doing, it can be simulated.

They are my favorite marine fish.

I had one for years that was a donation fish from a customer who ordered them on the internet and kept killing them. Poor thing had barely any top fin left and was so stressed he was almost pure white! He lived in my 200 gallon for about 3 years, regained full color and his long banner. A friend of mine ended up putting him in his 400 gallon. The fish is still alive and well. Considering it has had over 300lbs of live rock since I and then he took the fish in, and has had specialty food, he is thriving.

They are Hard to keep and most of that is down to tank condition's and being Shipped .They don't tolerate being Shipped very well unlike other fish and not many people get the tank in the Right condition to keep them.

I don't own one but i wanted one for my Reef tank but my Friend has a Moorish Idol in 100 gallons she has had him a year and from what i have read she is lucky as they are a very Difficult fish to keep.

Even experienced Marine Keepers have problems keeping them alive

I agree with Sean,

They are definitely for experts; they're very picky eaters. I've seen a few tanks at fancy Dr. Offices, but I'm certian they have people come in to attend to them.

they can only be kept by experts, so not many people can keep them.