I'm no expert on slang from the days of 1909, and I haven't read Aleister Crowley. But at a guess; could "Plaster of Paris affair" refer to the experiment the character is about to do? Plaster of Paris is the stuff used to immobilise a broken arm. It's fragile and not very durable. Or could it refer to the skull that the "surgeon" is going to operate on?
(Other thread-) "All out" could it be a metaphor from cricket?
Other phrases spring to mind- old-fashioned slang. eg: "All up" meaning there's no hope for him. "Done for" means no hope too. In the context "all out" perhaps is the same?
Other suggestions needed, I think!
Or does Plaster of Paris refer to the idea that the surgeon is trying to immobilise the whole body of the victim??
Interested in Language