Writing in TalkTull, Pamela suggests that Tull's is a variant of the world-famous Oberammergau Passion Play, performed every ten years by the entire population of the small German village, and by definition records the life of Jesus. I'd agree that the structure of 'A Passion Play' mirrors that of the Christian Passion play (incidentally, not specific to Oberammergau - it's an entire genre, performed in hundreds of locations for centuries). A traditional Passion play is indeed based on the life of Jesus, but I don't think the Tull Play is about Jesus, or even specifically religious. Ian took the format and generic title, and there are Christian references, but the story is largely his own invention.
Also in TalkTull, Timothy Foos offers a rather different view of The Play's theme: that the Passion Play is life itself, the story beginning at birth, with the silver cord being the umbilical cord.
Were one to accept that The Play is an abbreviation of the Chateau material, rather than a section of a larger work, Timothy's idea might indeed fit the context of the Chateau material.
Timothy suggests that the line 'the ice-cream lady wet her drawers' refers to the protagonist's mother, the wetting her drawers being her waters breaking. Timothy: "This gives credence to the implication that the ice cream lady wetting her drawers was a sacrifice, or unpleasant prerequisite to 'see you in the Passion Play.'"
There's a common impression that the ballerina on the album cover and the 'Lifebeats' & 'Hare...' films was Ian's girlfriend, or even Shona Learoyd (later Mrs. Anderson). The latter certainly isn't the case. Shona did appear on the WarChild album cover, as the Ringmistress, but didn't resemble the ballerina, whose name is Jane (I don't know her surname). I can't authoritatively dismiss the idea that Jane was Ian's girlfriend, but there's absolutely no reason to think she was, beyond rumour spread decades later, and Ian was still married to Jennie at the time.