Neil,
This seems subtle, and hence interesting. I will hazard a few questions which may be a bit off target, but I hope you will bear with me. I'm a mathematician by training and not very familiar with half-wave plates, so I'll take your word for it that they work as you describe.
1. Looking at your experimental setup, are you saying that the simpler experiment of passing n different photons through the detector has been done and does indeed change the angular momentum of the plate macroscopically?
2. Isn't there a "time factor" involved? It seems that if the angular momentum of the plate is changed macroscopically, it will be rotating, and the preferred axis will change, hence changing the interaction with subsequent photons more or less depending on how quickly they are passed through.
3. It seems that there would be a minimal possible angular momentum transfer, but I suppose the expectation values can change by arbitrarily small amounts?
4. Have you thought about decoherence rather than projection in this context?
5. Have you thought about the sum-over-histories approach in this context? The reason I ask is because the sum-over-histories approach is central to my own thoughts about quantum/theory quantum gravity. Also, since you mention superluminal communication, I will remark that I think causal cycles are not necessarily paradoxical; (see my essay On the Foundational Assumptions of Modern Physics for more details on these points.)
Thanks for the interesting read. Take care,
Ben Dribus