Ian Cutler and Gary Carpenter mention it but there is no evidence left of it other than than the 27 seconds we hear of its music in the trailer* from :39 to 1:09. See Steve P’s page for their recollections of it. (Search for “dream" on that page.) Who knows how long it really lasted. It was very expensive and difficult to put together musically and cinematically. It has never been seen by anyone not involved with its making.
The music was definitely recorded at Shepperton in post-production, says Gary, with just the musicians seen in the film.
Ian recalls that the sequnce seems to have had at least these features: “a kaleidoscope of images, a huge egg-shaped stone is revolving faster and faster. Also the woman in the churchyard who is feeding the baby has the egg in her hand and crushes it.”
Gary describes it as “phenomenally complex”… “like post-scoring an animation, it was so intricate. With “fades and dissolves and extensive use of library footage…” making it expensive.
In Hardy and Shaffer’s novelization, there is a dream that takes place the night before May Day, after Willow’s dance. (It can be found on pages 140-143 of the Crown, 1978 hardcover.) It is quite different from what Ian Cutler’s memories of it are, assuming it is the same dream. (I don’t remember any others.) No huge revolving egg-shaped stone nor the woman in the churchyard crushing the egg she held in her hand. The novel version talks about Howie seeing Rowan in an oak tree crying for help, someone named Beech attacking him with a claymore (sword), then Broom, then an army led by Lord Summerisle complete with pipes and in 18th century garb and look. They stop when he recites some lines from Lars Porsena of Clusium by Thomas Babington Macaulay, "By the nine gods he swore…." This holds them for a little while until Lord Summerisle cries out “Wrong, ridiculous, pathetic” and they attack again. Howie wakes up in a sweat soon after this.
The film version would have taken place as the Hand of Glory burned, Cutler says. In the novel the Hand of Glory scene takes place almost verbatim as the movie so Howie doesn’t sleep or dream then.
*I am assuming this trailer was what was used in theaters when it was released. I don't know if this was used in the US or the UK or both.